I have an A/C unit that was connected to the breaker box with 8 gauge wire and a 40 amp 240 volt breaker. The A/C unit is small and rated at 20 amps. Is there any significant risk in just replacing the 40 amp breaker with a 20 amp and not changing the wiring (would require breaking through drywall,etc.)?

No, but why bother?

When you say rated, what do you mean?

A/C units have a minimum circuit ampacity and a max fuse/breaker size.
What is the max breaker rating on your unit?

The minimum circuit ampacity is 13.0.
The max breaker rating is 20.0.

Thanks