I've upgraded a couple of times and frankly as long as you're not adding uncured rock you're good to go. All you're doing is increasing water volume so unless you're adding a crapload of new livestock to the new tank at the same time as you move the contents of the 90 into the bigger tank you're certainly not increasing the bioload. Even if the rock you have in tubs was dead you're still fine as you're just moving the same bioload from the 90 into a tank with more water. Its like doing a massive waterchange and everyone will be the happier for it (noting temp and salinity is the same of course).
edit: LOL, ok I just missed all of page 2 and basically repeated what sphelps said.
Carry on....
