I usually go cold-turkey when we make a big dietary change. I tell myself and family that we'll try it for six weeks, because that seems achievable. Then, if it's working, we continue.
And, yes, digestive enzymes were what I meant. We use the ones from Houston. No-Fenol, Peptizide and HN-Zyme Prime; one each with every meal. At first I opened them and sprinkled them on the first few bites of food, then I eventually convinced my son to chew them and now he swallows them before eating.
If he gets too many salicylate foods in one day, or has something with artificial ingredients, I give him two No-Fenols, as that seems to help combat the ill-effects.