Re: Was ANYTHING retained when the Enterprise was refit?
I'd be curious to know if anyone at the time considered saying it was a new and different starship Enterprise. Although, I could see them being uncomfortable with that idea because it meant either Kirk's Enterprise was destroyed sometime between TOS and TMP, or it was retired. And of course, there would be registry number issues because the new ship couldn't technically be NCC-1701. Guess it could've been NCC-1701-A, though.
But it's silly to rationalize the ship as a refit of the TOS Enterprise. It's just too different in size and proportion. No attempt was made by the designers (of the model and sets, not in-universe designers) to keep bits and pieces of the old look just as a nod to the idea of it being a refit.
I don't see why it couldn't be the same ship. History knows examples, where even a destroyed (or in this case, a sunken) ship has been rebuilt to look completely different than the original one. For example USS Merrimack was rebuilt as CSS Virginia:
"God runs electromagnetics by wave theory on Monday, Wednesday, and Friday,
and the Devil runs them by quantum theory on Tuesday, Thursday, and
-Sir William Bragg