You can change it much, but the key idea is to change it slowly, very slowly. About 0,2/0,3 units per 24h. E.g. you want to decrease pH from 8 to 6,5 and add very weak sulphuric acid (however, don't ever put sulphuric acid direct into your tank, add it into water first in another pot). You decrease pH about 0,3 units per day, so it takes about 5-7 days at least when you reach pH 6,5.
If fishes start to behave odd, then stop it - don't change pH at all until fishes are normal again. And after that, do it even more slowly.
Edit. I put some turf into one filter once and pH went down about 0,5-0,7 units during night. Didn't see any odd behaviour.. Probably pH changed smoothly and didn't affect fishes so much. 0,5 units isn't so much, some sensitivies fishes can react on it too, so it's always safe to change pH slowly.
