This is a follow-on to:
Does a caster under the effect of a True Polymorph spell retain their class levels?
Assume we have a wizard who wants to check out a cave system where he or she can't fit through (just as an example application of this), but he isn't a Druid, so can't WildShape to a form that's useful. However, said Wizard has True Polymorph at his disposal, so he can shapeshift into something usefully-sized that way, such as a kobold.
Can the wizard elect to polymorph himself (or someone else) into another form and then grant that form class levels in Rogue (or any other class) as part of the spell, or is that something that's strictly forbidden by RAW?
Best Answer
Core Rules
No
From the spell description p. 283
It explicitly states that creatures without challenge ratings have levels. This implies that creatures with levels do not have challenge ratings.
The spell only allows you to turn the target into a creature with a challenge rating - not a creature with levels.
For example, you can choose any of the humans in the monster manual, the priest, the assassin, the archmage etc. but you cannot choose a Human Wizard Level 19 because this creature has no challenge rating.
Optional Rules
Yes, but its not straightforward.
From p. 283 of the DMG for NPCs and Monsters with Classes respectively:
The older edition paradigm of CR=Level does not hold true in 5e.
It would take some considerable time to determine the CR of you Kobold + Wizard level monster and is likely to bog the game down dramatically if it happens during a session. If this is something that you want, I suggest the DM and the wizard decide on a few "go to" polymorphs and work them out in advance.
This appears to be how the Monster Manual Appendix B NPCs were assigned their CR.