I don't think it's so much as the established water that makes the balancing faster, it's more or less the AquaSoil + BB. However, I'd still add back at least 30 to 50% of the established water after the substrate swap.
The AS is just loaded with nutrients and when it gets in contact with water, it just wants to let out a lot. So you just have to let it run its course and reduce its impact by PWC's. Plus, the BB needs to re-establish on the substrate. I believe mulm could help though. So if you're removing some old substrate, leave some of that mulm on the bottom. Also the plants are coated with BB, so when your replant them, you're 'seeding' BB in the tank.
Do you have an API Freshwater Master Test Kit? This is the sure-fire way to know when everything is all said and done -- and safe to return the fish to the tank.