After you've been assigned as recipient, you cannot change the wish any more. Assigning the recipe "locks" the wish and lets other users know you're done editing it. You can’t unlock it yourself to avoid cheating: If you were able to unlock it, you would be able to see if the wish has been purchased and by whom. That would clearly ruin your surprise! However, if you want to change something, you can ask someone to remove you as a recipient provided the wish has not yet been bought.