“Don't try the paranormal until you know what's normal.” - Granny Weatherwax
Just to be sure:
While you use this item to gain XP during a quest, you still have to wait until the next Campaign Phase to be able to spend the XP right?
I always thought that you could buy replacement class skills on the spot (and as an overlord I would certainly allow it as a house rule). However, by RAW you are right: that hero should wait the next campaign step to spend the XP he just acquired.
By comparison, Avrel Wolrdwalker's Heroic Feat allows her to explicitly change her class skills selection on the spot.