With modern cars incorrect octane will decrease available power, as the knock detector reduces the spark's advance, but it won't otherwise hurt the engine.

Low-octane gas will cause problems in older carbureted cars, but most cars sold in the last thirty-or-so years will not have this problem.

Don't the cars just dial back ignition timing when they detect knock? Unless your car has no ability to dial back, maybe with a custom high-octane tune, regular fuel should still be safe to run.