If you've been feeding dry, it can take several cycles to get that cleared from his body, so it'd probably be fine to start tonight if you want to....No matter what though, you want to do this slowly....Not only because of the possible effect on his blood glucose, but because you don't want to upset his tummy and kick off a round of diarrhea from changing food too quickly
No more than 25% per day should be changed, and if that quickly causes problems, you should go even slower