They re-imagined the Enterprise, just like they re-imagined the Klingons.
Well, not really.
After all, they took care to show the old
ship as well, in that Rec Deck display of images. TOS wasn't supposed to be "remembered differently", or forgotten. Not in that particular respect anyway.
The thing about the TMP ship is that, backstage measurements aside, every part of her is bigger than the "original" TOS counterpart. The saucer could have been created by adding a new rim and bulging out the bulges a bit more; the secondary hull could have received a new surface layer of spaces; the neck could have been strengthened.
Basically, the old ship would fit inside the new one, save for the engines and the secondary hull bow.
The pic in the rec room was a cute little nod for fans, but that's about it. Gene Roddenberry himself suggested The Original Series as an inaccurate portrayal of the 23rd century in his novelization of The Motion Picture. And of course, he's on record that the Klingons "always" looked the way they did in TMP and beyond, and that's the way it was until DS9 made a joke of it and Enterprise's fan-wank-tastic final season that said otherwise.
The complete change in look from 60's TV (with randomly blinking coloured squares for buttons) to late 70's movie (with far more realistic interfaces) is subsitution, not a plausible in-universe evolution.