A229: Ask yer DM, but RAW, they'd lose the benefits until they meet the prerequisites again; much like using the chameleon's flotaing feat to qualify for a PrC.
More specifically, do you regain what you traded away temporarily?
I can see three ways this might work:
1. You lose the benefits of the level, but also regain whatever you had to trade away, treating the level as a normal one of that class.
2. You lose the benefits, and do not regain whatever you traded away, as though you had taken a gimped level of the class.
3. You lose all the benefits of the level, as though you had never taken it.
Number 3 seems the most consistent, but it's a weird corner case, so I thought I'd ask for clarification.