I'd like to make an observation. The following 3 things are different:
1. Advancing a Monster to make it tougher ( 'by HD')
2. Advancing a Monster by class levels (which might alter it's role, too)
3. Making a Monster into a Playable Character
If we assume that 4e MM has raw stat blocks like so (some things left out

Dire Wolf (CR3)
hp 45
AC 14 (...)
Attack: Bite +11 (1d8+10)
Abilities: Str 25, etc.
For 1, it's trivial. All you need are some guidelines about what a CR4, CR5, CR6, etc. creature should be able to do and upgrade Wolfie to match.
For 2, it's still pretty easy. Simply write up all the class levels, and add it's bonuses to Wolfie. For instance, if you give him 4 level of Fighter, Weapon Focus: Sharp Pointy Teeth, and Weapon Specialization in the same:
Dire Wolf (CR ???)
hp 79 = 45 + 34 (4 FTR w/ 17 CON)
AC 14 (...)
Attack: Bite +16 (1d8+12)
Abilities: Str 25, Dex 15, Con 17, etc.
Easy! Just treat all class features as additive, and it's done. Now, that will make CR more difficult; you'd have to look at the per-level guides to find the new CR (this could be hard, if you changed it's role by adding Warlock levels)
Option 3 is problematic, but it's already problematic for most creatures (How many pages of Savage Species were devoted soley to determing Level Adjustment of Creatures?) For making a monster a Playable Race, either the designers would craft a racial feature level set (like the base races) or the DM would have to do it.
Naturally, a 4e Savage Species could contain piles of Racial Level sets for off the wall creatures. I'd prefer that to trying to cram it into the core books.