The existing 8 gauge wire going to the breaker box cannot be changed (no access to box from attic). I guess the question is can the breaker itself be changed and if so, to a 20 or 30 amp?

Yes, you can downsize the breaker... the problems you may face are getting that #8 into the lug opening of a 20 amp breaker, I wouldn't use a 30 if the oven is rated 20. At the wall oven, depending on the termination box, the same problem may exist.

Add a #12 pig tail to the end of the #8 and connect to breaker.