Yeah, you'll have to re-learn all your distances, which is why switching from normal-distance balls to extra-distance balls (say, GI-D) to ultra-distance balls (say, GI-SD) and back can be problematic & confusing. However, I'd definitely recommend switching to extra-distance or ultra-distance balls; on some holes, they can make all the difference between a par and a birdie (or even a birdie and an eagle, like on Bethpage 13 and the other par 5s).