If I recall correctly the Stronglifts guy says you can switch over to dragon flags if the reverse crunches become too easy.
When I was doing Stronglifts I got to the point of doing the reverse crunches easily without a supporting weight very early and switched over to dragon flags. If you switch, BE CAREFUL! Make sure you are still warmed up. This shouldn't be much of a problem if you're doing the exercises in order but I was showing my brother how to do them at home from a cold start and injured my back. I was out of training and lifting for two weeks. Learn from my fail.