Alternate Form Spell Triggers Help

I have a spell which transforms a hero into something else, and it there are five levels to it. If the hero levels up in the spell which changes him into something else, then when the spell duration ends he does not turn back to what he was. Otherwise he does and it is fine.
I have tried to get rid of this bug using triggers to replace the unit with the original one after a set amount of time, but doing this makes the hero have to re allocate the spell levels he has chosen so far. This is a really difficult bug to fix and could really use some help with how i might stop this happening. Please suggest what might work...

Questions like this should be posted in the Map Development forum. Use the search funciton, this question has been answered before.