Villainous Competition XX​V: No Solicitors!

[Telonius]

I don't think the rules are completely clear on that part of that, but let's take an example. So say you have a creature that was originally a Magical Beast, who changes to a Dragon by gaining a template. The creature's Type changes from Magical Beast, to Dragon (Augmented Magical Beast). The creature's current features (i.e. attack bonus, saves, hit dice, skill points) don't change, but its traits do (it now has immunity to sleep and paralysis, etc).

Most templates make it explicit that the features don't change; they have text saying things like, "don't recalculate hit points, saves, skill points." That's just reiterating the general rule that features don't change. There are a few exceptions, like the already-mentioned skeletons and zombies, where the template specifically says to change the dice to d12s.

Now that the creature has a new Type, it's not totally clear what would happen in advancement. Personally I think that any racial hit dice would belong to the new type. It wouldn't make sense for it to gain hit dice of a Type that it no longer possesses; but the rules don't state this explicitly.

EDIT: Regarding CR, I'd think that the Magical Beast HD would count a +1 CR per 3, and any Dragon HD would count as +1 CR per 2. So if it's Magical Beast 9/Dragon 4 on a CR 1 base creature and a CR +1 template, it would be CR 7.