i.e. can a Thief level 5 who is
> now a Priest level 3 have 5 free actions (of a thiefish nature? (And apply
> the 5 on his initiative?) or must he forgo the thief free actions?

If PHB applies, he is now a 3rd level priest, he could use the thief abilities at the cost of all
priestly xp or appropriate punishment. I would surmise that only free actions of the priestly
class could be taken until he was a level 6 priest.( with similar penalty for using thiefly ability)