...and when even Stan can't understand why they did it, well, it says something.
(And yes, the letter-writer--or the editor--left out the e's: Espionage and Enforcement. Give yourself a No-Prize)
Oh, they gave an in-story reason for S.H.I.E.L.D.'s name change--after the original S.H.I.E.L.D. was infiltrated by rogue LMDs, it was disbanded, and the UN created a new agency that just happened to have the exact same acronym for a name. Because that way the UN didn't have to pay for new patches for all the uniforms, I guess. (Also, because trademark preservation is important, of course.)
But that's in-story. I've never heard a real-world reason why it was necessary or desirable to change what the damn initials stood for. Why, after 25 years, it was important to change the name that Stan and Jack came up with and that thousands of fans had memorized, and replace it with the obviously inferior one that Bob Harras (the S.H.I.E.L.D. writer at the time) or whoever came up with.
And it's now been 30 years with the "new" name. Aren't we due for some young turk to rename S.H.I.E.L.D., to mark their territory and leave their imprint in the Marvel Universe?
So, don't be surprised when a post-Secret Empire S.H.I.E.L.D. forms, calling itself the Sublime Holistic Interdisciplinary Emirate for Logical Destruction, or whatever. Come up with your own--it still won't be better than Stan and Jack's.
By the way, Wikipedia lists the following "related" organizations:
3 Related organizationsGet cracking, kids--replace all of those acronyms with hipper, "better" ones!!
3.1 A.R.M.O.R.
3.2 H.A.M.M.E.R.
3.3 S.P.E.A.R.
3.4 S.T.R.I.K.E.
3.5 EuroM.I.N.D. and S.H.A.P.E.
3.6 S.T.A.K.E.
3.7 S.W.O.R.D.
3.8 W.A.N.D.
From Avengers #20 (1999)