When an existing creature acquires a template, and that template changes the creature's hit dice, does this change the creature's existing hit points?
A 3rd level human cleric with no bonus from constitution has 8 + 2d8 hit points (full hit points at first level plus 2 more hit die). Let's say the hit die rolls were 4 and 6, so the cleric has 18 total maximum hit points. Now the cleric is infected with vampirism, which changes his type to undead and, according to the SRD, "
[i]ncrease[s] all current and future Hit Dice to d12s". So, now the vampire cleric has 3d12 hit dice instead of 3d8. Is there any adjustment to his current maximum hit points of 18, which were generated from d8s instead of d12s?
I don't off-hand know if there is some acquired template that might reduce current hit dice in a manner that would make a creature's maximum hit points prior to acquiring the template become an illegal value (too high) for the templated creature. If there is a case in which acquiring a template would downgrade the hit die, then it certainly would open up this situation. Wouldn't maximum hit points need to be rerolled or adjusted in some way?