asco wrote:
An inspection on our home (7years old) recently revealed that both our
air conditioning units have 40 amp circuit breakers instead of 30 amp
(the unit asks for 30 amps). How big of a deal is this?

Depends on whether or not you care about your AC unit being protected
from burning up under various fault conditions. See the post by Dan
Akers.

And if so, is
it an expensive thing to change?

No, it's an easy 5 minute DIY job. Don't be surprised if an electrical
contractor wants $75 or $100 to do it though.