If a critter already has class abilites and I add additional levels of the same class, do they stack or overlap?
Example: an old bronze dragon casts spells as a 11th lvl sorcerer. I add 3 sorcerer lvls. Does it now cast as a 14th lvl sorcerer?
Yes, if it's exactly the same class. But then it's associated, which increases CR +1 per +1 class level.
In the case of that old bronze dragon, you're better off aging it two categories (to ancient, as it would gain 4 sorcerer spellcasting levels), which is also CR +3 (if you'd wanted to add 3 sorcerer levels).
Right, that was just for example. The question was because I'm running an AoW game and am incorperating psionics. I'm changeing the Illithid in HoHR to the psionic variant. Mind flayers manifest powers as a 7th lvl psion telepath if I remember correctly. I wanted to know what would happen if I added more psion lvls.