Example in MM has a frost giant with his original (base) HD. It gives no examples of a monster with advanced HD.
Also, Original - "belonging or pertaining to the origin or beginning of something, or to a thing at its beginning". An advanced monster isn't the original. RAW it might be unclear if you're one of those people who just search for ways to break rules, but RAI and looking at the context it is clear, at least to me. But I doubt that I will convince you (BTW, I'm not saying that you're a munchkin or anything like that, just talking about people in general).