It doesn't change a thing. Hit Dices are a waste of ECL (except in monster classes, where you get class features). Assuming the character is level 20, so ECL 28 with the vampire template, he suddenly becomes ECL 48 and must go through 20 levels worth of experience without gaining anything from it (not counting the LA of the Sangrolu template, which is presumably negligible).
Honestly, immunity to turning is definitely a better idea. Furthermore it has already been done in the past, both in homebrews and official works, so it's not something without antecedence.