Due to budget reasons it is also entirely possible that those older starships did have exterior changes but couldn't be depicted in the cgi or blue screen shots of the 80s and 90s.
There were actually two IRL reasons why we didn't see any exterior modifications to all of the TOS-movie ships (Excelsior, Reliant, Grissom, Klingon BoP, etc.):
1. Any modifications made to those models to show that they were upgraded over time would have had to be removed whenever those models would be reused for future TOS films. Remember, the idea at the time was that TNG would be running concurrently with Kirk and co.'s movies, which they planned on continuing to make for the foreseeable future. That was too much of a pain to deal with and could have potentially damaged the models, so they went unchanged. So instead they just gave the impression in TNG that these ships were old with some interior upgrades (by using the Enterprise-D sets.)
2. Several times when we saw older designs in TNG (BoP, Spacedock, Regula One, K'T'ingas, etc.), they were just stock footage from the films.
My head canon is that there's a cut-off that decides whether a ship can be usefully upgraded into the 24th century, but it's not particularly related to the spaceframe or the drive systems. It's isolinear vs duotronic-based computers.
That's why we don't see Connies, but we do see Mirandas and Excelsiors usually with higher registry numbers. These are early-mid 24th century new builds with isolinear tech built-in, and therefore they support the latest LCARS systems.
Other systems are simple enough to upgrade, but you need to have the right computer framework to support them.
This is pretty much what I think as well. Using my Soyuz analogy, the 2XXX Excelsiors in TNG were the original duotronic-based ships, while the 4XXXX Excelsiors were the isolinear-based ships, despite the exteriors looking the same. Same with the 1XXX Mirandas and the 3XXXX Mirandas. During the period from the 2290's to the 2340's, it was probably easier to just use the same spaceframe with interior upgrades, rather than designing and building new spaceframes (at least until the 2350's when it appeared that Starfleet started making the new Galaxy-family-style designs.)
On a side note, I was looking forward to possibly seeing some Starfleet vessels in the Section 31 movie, considering that it took place in the 2320's, but that didn't happen. All things considered, it's probably a good thing in retrospect that they didn't have any, since the producers are nowhere near as anal-retentive about starship design lineages as I am, and I'm sure I would have been disappointed.