the effect is negative, but not overwhelming - it depends on how much effort you have put into the kid and whether he is just barely considering you or solidly committed - think of it as losing a few effort "points"
it is my impression that it becomes more of a negative if he doesnt have an offer on the table for a few cycles
so, if the guy becomes your backup plan and does still consider you after you pull the schollie at some point he will look elsewhere, and he will immediately become more receptive to other schools.....which makes sense, right?