Problem is, it wouldn't take minutes to replace the saucer. You'd have to pull the Galaxys out of the front-line to transfer to a starbase where the "combat-saucer" is stored. Then the civilians and non-essential personnel would need to be transferred to the starbase, with additional engineering/tactical personnel transferred to the ship, then the saucers exchanged, then
the ship would fly back to the front-lines.
You would also need to build dozens of saucers for each Galaxy, as of course who would the war be against? The Cardassians? The Romulans? The Klingons? The Borg? Or a myriad of other races. Saucers would have to be stored on all the borders, and you would need to keep the extra personnel required at these bases, unused for months or even years at a time.
Then, there are reportedly only 5 Galaxy-Class starships (though there appeared to be a few more constructed by the time of the Dominion War). That would mean a huge amount of resources dedicated to a relative few starships to create a few "uber-ships". The problem with uber-ships is they become phaser-magnets.
Increasing the firepower by a vast amount will not mean you can increase the shielding, and that's not taking into account the extra power-requirements that all these weapons will require. The warp-core will probably not be able to cope with the added workload, and you'd need additional power reactors in the saucer, reducing the weapons load. You also have to consider that the saucer on the Galaxy Class contains most of the living quarters and essential systems like sickbay, so the space you can use for additional weapons will be further restricted.
Starfleet would be best off constructing Akiras, Steamrunners, Nebulas and other ships that could be used on regular patrols and other non-war missions.