diff --git a/db/post_migrate/20251016010314_swap_columns_for_merge_requests_bigint_conversion_stage_one.rb b/db/post_migrate/20251016010314_swap_columns_for_merge_requests_bigint_conversion_stage_one.rb
new file mode 100644
index 0000000000000000000000000000000000000000..c9a5e6b86ef1a7e399f6fbbb8f89c6e1bcd04acd
--- /dev/null
+++ b/db/post_migrate/20251016010314_swap_columns_for_merge_requests_bigint_conversion_stage_one.rb
@@ -0,0 +1,94 @@
+# frozen_string_literal: true
+
+# This migration does not need to no-op when failed.
+# Type validation prevents accidental schema modifications.
+class SwapColumnsForMergeRequestsBigintConversionStageOne < Gitlab::Database::Migration[2.3]
+  include Gitlab::Database::MigrationHelpers::Swapping
+  include Gitlab::Database::MigrationHelpers::ConvertToBigint
+  include Gitlab::Database::MigrationHelpers::WraparoundAutovacuum
+
+  disable_ddl_transaction!
+  milestone '18.6'
+
+  TABLE_NAME = 'merge_requests'
+  COLUMNS = %w[assignee_id merge_user_id updated_by_id milestone_id source_project_id].freeze
+
+  INDEXES = %w[
+    index_merge_requests_on_assignee_id
+    index_merge_requests_on_merge_user_id
+    index_merge_requests_on_updated_by_id
+    index_merge_requests_on_milestone_id
+    idx_merge_requests_on_source_project_and_branch_state_opened
+    index_merge_requests_on_source_project_id_and_source_branch
+  ].freeze
+
+  FOREIGN_KEYS = %w[
+    fk_6149611a04
+    fk_641731faff
+    fk_6a5165a692
+    fk_ad525e1f87
+    fk_source_project
+  ].freeze
+
+  def up
+    return if skip_migration_as_bigint_columns_non_exist || skip_migration_as_bigint_columns_type_non_match('bigint')
+
+    swap
+  end
+
+  def down
+    return if skip_migration_as_bigint_columns_non_exist || skip_migration_as_bigint_columns_type_non_match('integer')
+
+    swap
+  end
+
+  def swap
+    unless can_execute_on?(:merge_requests)
+      raise StandardError,
+        "Wraparound prevention vacuum detected on merge_requests table" \
+          "Please try again later."
+    end
+
+    # rubocop:disable Migration/WithLockRetriesDisallowedMethod -- custom implementation
+    with_lock_retries(raise_on_exhaustion: true) do
+      COLUMNS.each do |column|
+        swap_columns(TABLE_NAME, column, convert_to_bigint_column(column))
+      end
+
+      reset_all_trigger_functions(TABLE_NAME)
+
+      INDEXES.each do |index|
+        bigint_idx_name = bigint_index_name(index)
+        swap_indexes(TABLE_NAME, index, bigint_idx_name)
+      end
+
+      FOREIGN_KEYS.each do |foreign_key|
+        bigint_fk_temp_name = tmp_name(foreign_key)
+        swap_foreign_keys(TABLE_NAME, foreign_key, bigint_fk_temp_name)
+      end
+    end
+    # rubocop:enable Migration/WithLockRetriesDisallowedMethod
+  end
+
+  def tmp_name(name)
+    "#{name}_tmp"
+  end
+
+  def skip_migration_as_bigint_columns_non_exist
+    unless COLUMNS.all? { |column| column_exists?(TABLE_NAME, convert_to_bigint_column(column)) }
+      say "No conversion columns found - migration skipped"
+      return true
+    end
+
+    false
+  end
+
+  def skip_migration_as_bigint_columns_type_non_match(column_type)
+    unless COLUMNS.all? { |column| column_for(TABLE_NAME, convert_to_bigint_column(column)).sql_type == column_type }
+      say "Columns are converted - migration skipped"
+      return true
+    end
+
+    false
+  end
+end
diff --git a/db/post_migrate/20251016010315_drop_tmp_bigint_indexes_and_fks_on_merge_requests_stage_one.rb b/db/post_migrate/20251016010315_drop_tmp_bigint_indexes_and_fks_on_merge_requests_stage_one.rb
new file mode 100644
index 0000000000000000000000000000000000000000..3cc12eade88e405759837d4b42fe359a8d2196c7
--- /dev/null
+++ b/db/post_migrate/20251016010315_drop_tmp_bigint_indexes_and_fks_on_merge_requests_stage_one.rb
@@ -0,0 +1,160 @@
+# frozen_string_literal: true
+
+class DropTmpBigintIndexesAndFksOnMergeRequestsStageOne < Gitlab::Database::Migration[2.3]
+  include Gitlab::Database::MigrationHelpers::ConvertToBigint
+  include Gitlab::Database::MigrationHelpers::WraparoundAutovacuum
+
+  disable_ddl_transaction!
+  milestone '18.6'
+
+  TABLE_NAME = 'merge_requests'
+  COLUMNS = %w[assignee_id merge_user_id updated_by_id milestone_id source_project_id].freeze
+
+  INDEXES = [
+    {
+      name: 'index_merge_requests_on_assignee_id',
+      columns: [:assignee_id_convert_to_bigint]
+    },
+    {
+      name: 'index_merge_requests_on_merge_user_id',
+      columns: [:merge_user_id_convert_to_bigint],
+      options: { where: "merge_user_id_convert_to_bigint IS NOT NULL" }
+    },
+    {
+      name: 'index_merge_requests_on_updated_by_id',
+      columns: [:updated_by_id_convert_to_bigint],
+      options: { where: "updated_by_id_convert_to_bigint IS NOT NULL" }
+    },
+    {
+      name: 'index_merge_requests_on_milestone_id',
+      columns: [:milestone_id_convert_to_bigint]
+    },
+    {
+      name: 'idx_merge_requests_on_source_project_and_branch_state_opened',
+      columns: [:source_project_id_convert_to_bigint, :source_branch],
+      options: { where: "state_id = 1" }
+    },
+    {
+      name: 'index_merge_requests_on_source_project_id_and_source_branch',
+      columns: [:source_project_id_convert_to_bigint, :source_branch]
+    }
+  ].freeze
+
+  FOREIGN_KEYS = [
+    {
+      source_table: :merge_requests,
+      column: :assignee_id_convert_to_bigint,
+      target_table: :users,
+      target_column: :id,
+      on_delete: :nullify,
+      name: :fk_6149611a04
+    },
+    {
+      source_table: :merge_requests,
+      column: :updated_by_id_convert_to_bigint,
+      target_table: :users,
+      target_column: :id,
+      on_delete: :nullify,
+      name: :fk_641731faff
+    },
+    {
+      source_table: :merge_requests,
+      column: :milestone_id_convert_to_bigint,
+      target_table: :milestones,
+      target_column: :id,
+      on_delete: :nullify,
+      name: :fk_6a5165a692
+    },
+    {
+      source_table: :merge_requests,
+      column: :merge_user_id_convert_to_bigint,
+      target_table: :users,
+      target_column: :id,
+      on_delete: :nullify,
+      name: :fk_ad525e1f87
+    },
+    {
+      source_table: :merge_requests,
+      column: :source_project_id_convert_to_bigint,
+      target_table: :projects,
+      target_column: :id,
+      on_delete: :nullify,
+      name: :fk_source_project
+    }
+  ].freeze
+
+  def up
+    vacuum_detection
+    return if skip_migration_as_bigint_columns_non_exist || skip_migration_as_bigint_columns_type_non_match('integer')
+
+    INDEXES.each do |index|
+      remove_concurrent_index_by_name(TABLE_NAME, bigint_index_name(index[:name]))
+    end
+
+    # rubocop:disable Migration/WithLockRetriesDisallowedMethod -- custom implementation
+    with_lock_retries(raise_on_exhaustion: true) do
+      FOREIGN_KEYS.each do |foreign_key|
+        remove_foreign_key_if_exists(
+          foreign_key[:source_table],
+          foreign_key[:target_table],
+          name: tmp_name(foreign_key[:name]),
+          reverse_lock_order: true
+        )
+      end
+    end
+    # rubocop:enable Migration/WithLockRetriesDisallowedMethod
+  end
+
+  def down
+    vacuum_detection
+    return if skip_migration_as_bigint_columns_non_exist || skip_migration_as_bigint_columns_type_non_match('integer')
+
+    INDEXES.each do |index|
+      options = index[:options] || {}
+      add_concurrent_index TABLE_NAME, index[:columns], name: bigint_index_name(index[:name]), **options
+    end
+
+    FOREIGN_KEYS.each do |fk|
+      add_concurrent_foreign_key(
+        fk[:source_table],
+        fk[:target_table],
+        column: fk[:column],
+        target_column: fk[:target_column],
+        name: tmp_name(fk[:name]),
+        on_delete: fk[:on_delete],
+        validate: true,
+        reverse_lock_order: true
+      )
+    end
+  end
+
+  def tmp_name(name)
+    "#{name}_tmp"
+  end
+
+  def skip_migration_as_bigint_columns_non_exist
+    unless COLUMNS.all? { |column| column_exists?(TABLE_NAME, convert_to_bigint_column(column)) }
+      say "No conversion columns found - migration skipped"
+      return true
+    end
+
+    false
+  end
+
+  def skip_migration_as_bigint_columns_type_non_match(column_type)
+    unless COLUMNS.all? { |column| column_for(TABLE_NAME, convert_to_bigint_column(column)).sql_type == column_type }
+      say "Columns are converted - migration skipped"
+      return true
+    end
+
+    false
+  end
+
+  def vacuum_detection
+    return if can_execute_on?(:merge_requests)
+
+    raise StandardError,
+      "Wraparound prevention vacuum detected on merge_requests table" \
+        "Please try again later."
+  end
+end
diff --git a/db/schema_migrations/20251016010314 b/db/schema_migrations/20251016010314
new file mode 100644
index 0000000000000000000000000000000000000000..267b780819c212e49f2cffe369af43e487e81706
--- /dev/null
+++ b/db/schema_migrations/20251016010314
@@ -0,0 +1 @@
+472f6d28aca8116b1f5fc6072f6f4cdcf41c72503104e186b1fc65db1949598c
\ No newline at end of file
diff --git a/db/schema_migrations/20251016010315 b/db/schema_migrations/20251016010315
new file mode 100644
index 0000000000000000000000000000000000000000..b90fe8e4c87cda771fbd4bbb97d2cb90309877fe
--- /dev/null
+++ b/db/schema_migrations/20251016010315
@@ -0,0 +1 @@
+4a87f1585b6288a912bf6081d7d8bf3e732f32d28951d7cac0f0b0ab95afe4dd
\ No newline at end of file