I'm not sure on that one, I'm pretty sure Blizzard changed that for both dots and hots.Originally Posted by Spots
As a test put rejuv on yourself and see how much it ticks in caster form and how much in moonkin (if you have improved moonkin aura of course). It does tick for more in moonkin form, my guess dots use your current stats and not the stats at the start.
They did that with buffs like earth shield too, pre-trinketing for buffs wasn't intended use of game mechanics I guess.