diff --git a/db/post_migrate/20250829061951_finalize_merge_request_id_bigint_conversion.rb b/db/post_migrate/20250829061951_finalize_merge_request_id_bigint_conversion.rb
new file mode 100644
index 0000000000000000000000000000000000000000..088210d07b33235189042f47bc33eab03f79306f
--- /dev/null
+++ b/db/post_migrate/20250829061951_finalize_merge_request_id_bigint_conversion.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class FinalizeMergeRequestIdBigintConversion < Gitlab::Database::Migration[2.3]
+  disable_ddl_transaction!
+  restrict_gitlab_migration gitlab_schema: :gitlab_main
+
+  milestone '18.4'
+
+  COLUMNS = %i[latest_merge_request_diff_id assignee_id author_id id last_edited_by_id merge_user_id milestone_id
+    source_project_id target_project_id updated_by_id].freeze
+
+  def up
+    ensure_backfill_conversion_of_integer_to_bigint_is_finished(
+      'merge_requests',
+      COLUMNS
+    )
+  end
+
+  def down; end
+end
diff --git a/db/post_migrate/20250829063051_sync_bigint_indexes_creation_on_merge_requests_stage_one.rb b/db/post_migrate/20250829063051_sync_bigint_indexes_creation_on_merge_requests_stage_one.rb
new file mode 100644
index 0000000000000000000000000000000000000000..b8310a59e3ea848c8412aad4e2e7a48854c085d6
--- /dev/null
+++ b/db/post_migrate/20250829063051_sync_bigint_indexes_creation_on_merge_requests_stage_one.rb
@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+class SyncBigintIndexesCreationOnMergeRequestsStageOne < Gitlab::Database::Migration[2.3]
+  include Gitlab::Database::MigrationHelpers::ConvertToBigint
+
+  disable_ddl_transaction!
+  milestone '18.4'
+
+  TABLE_NAME = 'merge_requests'
+  BIGINT_COLUMNS = [
+    :assignee_id_convert_to_bigint,
+    :merge_user_id_convert_to_bigint,
+    :updated_by_id_convert_to_bigint,
+    :milestone_id_convert_to_bigint,
+    :source_project_id_convert_to_bigint
+  ].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
+
+  def up
+    return if skip_migration?
+
+    # rubocop:disable Migration/PreventIndexCreation -- bigint migration
+    INDEXES.each do |index|
+      options = index[:options] || {}
+      add_concurrent_index TABLE_NAME, index[:columns], name: bigint_index_name(index[:name]), **options
+    end
+    # rubocop:enable Migration/PreventIndexCreation
+  end
+
+  def down
+    return if skip_migration?
+
+    INDEXES.each do |index|
+      remove_concurrent_index_by_name TABLE_NAME, bigint_index_name(index[:name])
+    end
+  end
+
+  private
+
+  def skip_migration?
+    unless conversion_columns_exist?
+      say "No conversion columns found - migration skipped"
+      return true
+    end
+
+    false
+  end
+
+  def conversion_columns_exist?
+    BIGINT_COLUMNS.all? { |column| column_exists?(TABLE_NAME, column) }
+  end
+end
diff --git a/db/post_migrate/20250829064230_add_bigint_foreign_keys_on_merge_requests_stage_one.rb b/db/post_migrate/20250829064230_add_bigint_foreign_keys_on_merge_requests_stage_one.rb
new file mode 100644
index 0000000000000000000000000000000000000000..470c99b6059211ff3622f192541f96e5d02f05cf
--- /dev/null
+++ b/db/post_migrate/20250829064230_add_bigint_foreign_keys_on_merge_requests_stage_one.rb
@@ -0,0 +1,96 @@
+# frozen_string_literal: true
+
+class AddBigintForeignKeysOnMergeRequestsStageOne < Gitlab::Database::Migration[2.3]
+  disable_ddl_transaction!
+  milestone '18.4'
+
+  TABLE_NAME = 'merge_requests'
+  COLUMNS = %i[assignee_id merge_user_id updated_by_id milestone_id source_project_id].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
+    conversion_needed = COLUMNS.all? do |column|
+      column_exists?(TABLE_NAME, convert_to_bigint_column(column))
+    end
+
+    unless conversion_needed
+      say "No conversion columns found - no need to create bigint FKs"
+      return
+    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: false,
+        reverse_lock_order: true
+      )
+
+      prepare_async_foreign_key_validation fk[:source_table], fk[:column], name: tmp_name(fk[:name])
+    end
+  end
+
+  def down
+    FOREIGN_KEYS.each do |fk|
+      remove_foreign_key_if_exists(
+        fk[:source_table],
+        fk[:target_table],
+        name: tmp_name(fk[:name]),
+        reverse_lock_order: true
+      )
+
+      unprepare_async_foreign_key_validation fk[:source_table], fk[:column], name: tmp_name(fk[:name])
+    end
+  end
+
+  private
+
+  def tmp_name(name)
+    "#{name}_tmp"
+  end
+end
diff --git a/db/schema_migrations/20250829061951 b/db/schema_migrations/20250829061951
new file mode 100644
index 0000000000000000000000000000000000000000..ca70f28c9dbb86a6405d740426f726c8f2811726
--- /dev/null
+++ b/db/schema_migrations/20250829061951
@@ -0,0 +1 @@
+256e3a86bb6c122799d7e536bff4c968bb1b82a9b06de1926c84a080c496adf0
\ No newline at end of file
diff --git a/db/schema_migrations/20250829063051 b/db/schema_migrations/20250829063051
new file mode 100644
index 0000000000000000000000000000000000000000..63bd86d1fef1777ad3dabc49a62e55f76fff78fd
--- /dev/null
+++ b/db/schema_migrations/20250829063051
@@ -0,0 +1 @@
+34a13029f9c035dce82729fdbb2b9599ce011991769e4d232e42c8b2efb29881
\ No newline at end of file
diff --git a/db/schema_migrations/20250829064230 b/db/schema_migrations/20250829064230
new file mode 100644
index 0000000000000000000000000000000000000000..d5fc3b8cfbfbcb20df065f85c4a39b7b07d7fa3d
--- /dev/null
+++ b/db/schema_migrations/20250829064230
@@ -0,0 +1 @@
+59faeadec58fc5f9c58e7dc6800eeec2e802b37f05f6e96663438b0b55ee86eb
\ No newline at end of file