Generations shows the most famous example of the Galaxy's level of customization. Stellar Cartography was a room that was the height of about three decks, but there's no way you could install that room without a somewhat major overhaul. That is, unless you have some sort of module configuration to simply take out and replace sections of the ship as needed.
On the other hand, stellar cartography can easily be explained as part of the Enterprise-D from the moment it was launched.
I do recall Geordi complaining at one time or another about shunting power from and to various parts of the ship (SC was included because it had to be shut down). The only reason it was never probably seen during TNG run were budgetary constraints (but it was mentioned).
Don't forget that the crew of Voyager constructed the Astrometrics lab in their own little ship.
A piece of equipment far more sophisticated compared to stellar cartography, and it only had a really large view-screen while occupying much less space.
While I do find SC better in terms of being literally surrounded by stars as a pretty good place to work in, the Astrometrics lab can operate with several individuals in it on same or various aspects.
As well, in "Liasions," the dialogue specifically states that Deck 8 was empty in case the ship would need extra space to be configured accordingly for any mission. As far as we know, there could be other decks on the Enterprise-D that were similarly designated, and it wouldn't be far-fetched to believe that future Galaxy-classes were refitted for the Dominion War. Say, a room as large as the 3-deck tall Stellar Cartography housing a large power core for phasers or shields, for instance.
Agreed, although, deck-8 of the Enterprise-D also contains the battle-bridge and officers quarters.
As for during/post Dominion War that the Galaxy classes were refitted with extra power cores for phasers and shiels - that was mostly fan idle speculation lacking in evidence.
If anything, the off-screen comments and interviews by those who worked on DS9 said that most of the Galaxies in battle were unfinished ones ([featuring only necessary crew quarters, functioning weapons/computers/engines] a far-fetched statement in my personal perception because I find it idiotic that it would take years for SF to build those ships in the first place for a technologically advanced organization as the Federation [too many real-world references are used to connect Trek and our time when in fact it's bound to fail once we take into consideration the fact that we are centuries apart in not just technology - the Federation gaining technological superiority in just 300 years compared to it's neighbors that were in space for centuries prior to them - but also perceptions]).
Besides, all ship systems (incl. weapons) have the warp core for main source of energy. If you will upgrade anything, then focus on the main source of energy to boost shield integrity and increase the phaser output, not to mention upgrading the EPS grid (that's how it was done on-screen to my recollection and not through the use of extra power cores).
The Galaxy classes likely underwent a smaller re-fit just before the war to incorporate modified anti-Borg tech the Defiant used so they can damage Dominion ships properly and their shields to actually keep Dominion weapons at bay just like with other weapons.
Post-war ... there is a possibility they could be brought on par with the Sovereign class.
But so can smaller ships such as the Intrepid.
The Prometheus was extremely powerful and likely had similar specs like the Sovereign class (regenerative shields, ablative hull armor, MVAM - and was able to roast both SF and Romulan ships with a few hits) and the Intrepid is similar in size.
Depending on what evidence you want to use as well (since you cited burst-fire launchers, a term that wasn't used in the show but was used in various books), the TNG Tech Manual says the Enterprise-D was launched with 35% of its interiors as customizable space so that the ship could be adapted with modules for almost any mission imaginable.
Books (as good as they are) are not part of canon.
'Personal canon' is not really taken into consideration since I'm trying to remain with what was established on screen and go from there.
However, to draw a comparison, Voyager easily used this same ability with their torpedo launchers in the novels as well.
There's also the Nebula. Ships of the class can be equipped with special pods depending on the purpose of the vessel, ie the big sensor pod or the big weapons pod or whatever other pods are out there. We've never seen this design element used in most other ships after it save for the itty bitty Danube, and yet the Nebula is one of the most often-seen classes in all of Trek, so something about that class *must* be working right.
Same with the Intrepid. Bioneural gel packs are a milestone in computer technology. They've been pretty dependable for Voyager and there've been no complaints about the technology mentioned in DS9. Yet we've no evidence that ships like the Sovereign and the Prometheus use that particular system.
Again, I'm not saying the Intrepid doesn't have a module configuration, it's just that it wasn't mentioned on Voyager, whereas the Galaxy was. Just because it's on one class doesn't necessarily mean it's on the other, and even outright we've seen the Intrepid perform missions differently than the Galaxy. Different ships have different purposes.
Nebula class ship has a specific pod that was obviously intended on being changed if the situation required it.
Although, I would also speculate that most Nebula's retain their pods as they got them at the first day of launch, unless of course a major conflict ensues and more combat oriented ships are needed.
In which case a 'combat pod' would be more useful since it could hold large amount of torpedoes (although I must admit that putting them in such 1 place and a nicely nested hit could end in a VERY undesirable result
).
As for the gel-packs...
Well, I see no reason why other Federation ships would not be using them in the post war era (when it was much easier to conduct larger upgrades without rushing things too much).
The computer systems on older ships would have to be modified of course to accept them.
Most of the Jeffries tubes and other areas on other Federation ships have similar/same computer access ports that could be modified to accept the gel-packs.
But it becomes more agile and faster, I bet it could even keep up with an Intrepid after getting rid of that saucer
More agile might very likely be the case when the Galaxy is separated
(a procedure primarily utilized in emergency situations anyway ... otherwise, why would SF send fully connected Galaxy class ships into battle?), but 'faster' is highly questionable since all SF ships appear to be equally fast under impulse.
