I'm pretty sure a logical, sensible argument can be made for why 40+ year old Enterprise was delegated to training status, with decommissioning looming in the near future, not the least of which being the upcoming Excelsior class Enterprise (almost certainly on the construction schedule by the time of TWOK-TSFS-TVH, pending the results of the tests on the Excelsior itself). The only spanner in the works being when Kirk stole the Enterprise and destroyed her a few years ahead of Starfleet's planned decommissioning date, prompting the early decommissioning of the Yorktown and recommissioning her, for the last few years of her planned service life, as the Enterprise-A.
As for uniform changes, get serious. Uniforms in the real life military get changed by headquarters bigwigs all the time, sometimes on a whim (ex: All Army troops can now wear black berets for no other reason that the Secretary of the Army under Clinton said so, which the Rangers didn't find the least bit amusing, prompting them to change to tan berets). Translate this to Star Trek, and I imagine it wasn't all that endearing for the experienced personnel when, around the time of "The Corbomite Maneuver", they suddenly had to go back to black collars, like they wore in the Academy. And let's not even discuss the TMP jammies...