In general it’s a no-no. As an example depending on the course CS may be faster than AS. So you would need to bump to an ST or SP class that your car is classed in.
Do we want an intermediate PAX class?
It’s not uncommon for someone to run a car in the wrong class to get finishing points for the year and they just want to try out a different car, but it’s usually a onetime thing and no one in the class cares.
I don’t care if you run AS, but with only 1 other car in AS, I may bump to X.