Rested XP is for the character, not class, and is a straight %. So if you have 200% of rested XP and switch to a level 1 class and use it all up in a few levels, it's gone. The good news is rested XP makes very little difference in the end. Fates (which will be the majority of your XP after 34) and quests don't use up any. Only killing mobs does.
Wow, well that sucks. It was helping me level quite fast during fate grinding.