This is from Mousesavers.com...a truly worthwhile bit of information!
A neat thing about leftover days on a child's Hopper pass: if the child turned 10 since you bought the pass, he or she will need an "adult" pass to enter the theme parks. In these circumstances, the child's pass may be presented at any Guest Services location, in person only, to have it exchanged for the required "adult" pass -- at no additional charge! This only applies to partially used passes.
A neat thing about leftover days on a child's Hopper pass: if the child turned 10 since you bought the pass, he or she will need an "adult" pass to enter the theme parks. In these circumstances, the child's pass may be presented at any Guest Services location, in person only, to have it exchanged for the required "adult" pass -- at no additional charge! This only applies to partially used passes.