I wouldn't worry about saving all the water. Out of curiosity, how big is the tank in question? Do you have a smaller tank you can put the fish in while you set up the big tank again, or will they be living out of a bucket?
What were/are you doing to adjust your pH? You can use a buffering solution that will change the parameters right away (note that I'm not talking about the ones that you buy in the store, since I don't know what's in them -- lots of folks roll their own with commonly (cheaply) available ingredients).
I would generally treat a tank-moving exercise about the same as I would introducing new fish to the aquarium (i.e. aclimitize the water temp, and if you can, slowly mix tank water with the water the fish are in till it's mostly tank water)
-Rick (the armchair aquarist)