Can/should I replace this 50A breaker with a 15A breaker?

I’m the recent owner of a 27yr old house. I’m looking to find more power for the basement that I’m finishing. There’s a 50A circuit going to one of the outdoor AC units – but the info sticker on that unit says it should be connected to a 15A breaker only (if I’m reading it right).

The 50A breaker is in the 2nd image – top left.

The question is – can I just replace the 50A breaker w/ a 15A (double one) and leave the heavy gauge wire in place? If so, is this a simple swap?

Alternatively, I’d prefer to actually run that 50A to a subpanel & use that to split the basement into 3 zones, but if I did that I don’t know how I’d power the A/C, since it requires 2 slots (and the entire panel is full).