Trying to switch from Enfamil Premium Infant to Gentleease at 3 weeks?

When I switched my son to gentleease, we alternated bottles. the first gentleease the second the old forumla the third gentleease... if you get that. it helps their tummys adjust. you do that for 10 bottles at least! then fully be on gentleease. my son occasionally spits up. but gentleease does make him a little constipated maybe 2 times a week.