Given the ease in which a Galaxy class ship can detach the saucer and given that the engineering hull is a complete starship in its own right, would it be cost effective for Starfleet to maintain a dedicated weapons platform that could replace the regular saucer in time of war?
In just minutes you could have the regular saucer detached and replaced with a platform that could be carried into battle with hundreds, if not thousands of torpedoes, pulse phasers, and all other sorts of weapons.
The platform could even be outfitted with larger impulse engines than the regular saucer to increase sublight manueverabilty.
I've thought of this sort of thing in the past, but not in the sense you discuss it here.
One thing that's clearly established in the "tech manual" is that the Galaxy p-hull is a superstructure with a lot of little modules suspended inside of it (they even talk about how the bits and pieces are attached to the structure). The point of this, from a show-production standpoint, was to allow the interior of the ship to be redesigned and even redefined from time to time without necessarily implying that the ship had to go through major redevelopment. They beam in, and beam out, internal modules as necessary.
SO... as far as the INTERNALS of the galaxy-class p-hull being changed... that's easy and cost-effective. External modifications are quite a bit more challenging.
I mention this because I've thought of alternative internal configurations for this same primary hull. And the three which seemed most plausible were:
1) "Aircraft carrier"... basically convert massive amounts of the interior into additional hangar space. (Yeah, the mostly-unseen main bay was big, but it could be a lot bigger!) The additional firepower would be provided by dedicated gunships... but this would do basically what you describe.
2) "Colonial transport"... massive cargo-bays and short-term-occupancy "bunkrooms" (along with increased life-support capacity).
3) "Mobile Starbase"... in this case, the saucer would have the exploration facilities removed and it would be flown to a specific location and dropped off, providing an easily relocatable base-of-operations, either for an area which was of only temporary interest, or to help secure a location prior to construction of a permanent facility.
The thing about those is that it's not a matter of STRUCTURAL CHANGES to the existing Galaxy primary hull.
Now, if you went for a totally different primary hull, you'd probably need to redefine elements of the design of the secondary hull as well... since I'm sure it's designed to be most efficient, strongest, etc, with the intended structural configuration.
I like it much more (in a combat-related role) as a carrier anyway...