We switched from an electric to a gas range 3 years ago. The 50-amp breaker for the range has been off since. We are considering putting in a supplementary A/C system for the 2nd floor. Our breaker panel has no empty slots. Could I use the 50-amp breaker to feed a sub-panel for the new A/C system?

Depending on your panel type, and current breaker arrangement, you can "almost" always make room. But to answer your question. Yes you can.

If #6 wire, I would have 60 Amps feeding to upstairs subpanel. May qualify as disconnect to air handler.

I'm curious to know your A/C specs?