The Galaxy class is supposed to have a 100 year lifespan, so you would assume that it was designed with significant upgrades in mind. I also doubt there would be a big difference if any between the Galaxy & Sovereign classes as they were only launched several years apart & upgrades would be compatible with either ship.
I have always thought of the Sovereign as a replacement for the Excelsior class rather than the Galaxy. To me it would make more sense for Starfleet to have designed it to replace a much older class ship rather than a relatively new class.
Also it is likely the Sovereign would have been in the design phase when the Galaxy was launched.