It never struck me as a believable upgrade.
Look at the upgrade of the Constitution, the old nacelles were taken off and replaced with newer nacelles, this would presumably allow for faster speeds/efficiency. Adding a 3rd nacelle, which looks just like the existing decades-old nacelles by that time always seemed odd.
The big gun was perfectly plausible, if enemy shield technology had been upgraded to the point that antimatter weapons weren't enough any more, you're either looking at a new type of torpedo (quantum), or something else, to replace photons as the primary heavy weapon.
If you imagine all new Starfleet ships as being built around a similar style cannon, it's not that difficult to imagine them finding a way to add one to a ship like the Galaxy class, that still has a lot of years service left.