RolePlayingMaster does it
RolePlayingMaster does this very well.
It comes with Jamis Generators, and you can create all the creatures you want.
From there, you can change virtually anything, from adding templates, to changing the race, to adding, changing or removing any class levels.
Everything is fully recalculated when you do anything like this. It even remembers the base ability rolls, so that if you change the race, the abilities are adjusted according to the correct racial adjustments. This includes races which have low standard ability adjustments, utilizing a special adjustment formula from the DMG.
You can, as an example, see which feats were chosen, as opposed to gained from race or class levels, and make appropriate manual adjustments if you end up with too many, as a result of class level editing.