One thing which puzzles me is the general lack of other Constitution refits out there.
I think it's just that they (the studio) reserved the hero ship for 1701, that's all. And when they designed it, I don't think they went "oh, we should make it so that it's physicall possible in-universe to have this upgrade." They just designed something that looked the way they wanted.
I mean, if they had kept Ken Adam's design for Planet of the Titans, for the same reasons, is there a way we could fit that with the original design in-universe ?
Maybe some way a devotee could rationalize it, but clearly that kind of continuity was not a factor in Kaufman's thinking. One story is that he wanted to kill the whole crew off up front and do it with all new characters, but I've often thought that was just an exaggeration of the 'Kirk goes missing for most of the movie' storyline they went with (apparently because they were unsure about making a deal with Shatner and keeping him for the lead.)
I do think it funny that the most interesting visual concept from Kaufman was the idea you could look down from the bridge and see into lower decks, most of the way into the saucer ... apparently they've done some kind of low-rent rip on that idea for the new movie -- though whether Chambliss thought it up or somebody showed him some TITANS sketches, i do not know ... this is the guy who put all the reflective surfaces into TREK09 because (in his words, my italics for points of stunned disbelief at his statements):
One of the first things I wanted to do in terms of materials had to do with reflections. In the Sci-Fi that I looked at to catch up – because this has never been my genre of moviegoing – I noticed everything for years had been molded plastics and leathers and there haven’t been reflective surfaces and transparencies. I thought of the foils that NASA has been using on the sides of their ships forever, but that kind of thing hadn’t ever made its way into the sci-fi movie design lingo yet. I felt I should introduce that here, that it would be interesting to bring that in on the bridge, as well as other sets on the Enterprise.
Based on his statements, I assume he saw zero movies with space helmets, since they ALL have reflections in them. He didn't see FIRST CONTACT since the Phoenix is pretty much all metal foil (it's the only actual Trek model I've held in my hand, this one I KNOW.) he didn't see TFF because you've got the actors reflected in monitor screens ... the list goes on and on.