It’s not worth trying to convince such people, the chance of success is very low and such discussions tend to get very uncomfortable very fast. Once you’ve established that someone supports authoritarian dictatorship, your engagement should be restricted to telling them such views are unacceptable in the space (if applicable) or correcting errors of fact for any onlookers.

But if you don’t already know the person’s views, it seems unwise to assume that “old person = supports the old regime,” since this will result in you missing opportunities to connect with potential allies.