Repurpose unused 50 amp hot tub panel for 30 amp hvac?
You can reuse all the #6 wire and old conduit that you please to use.
There is no reason to rip out or change any wires (and some reason not to: future expansion.
ONE of the breakers needs to be 30A.
You need to change either the main-panel breaker to 30A, or the breaker in this mini-panel/disconnect switch.   No need to change both.   If you change the mini-panel breaker, then the run to the main panel needs to continue to be #6/8/whatever it is now.
That Challenger breaker must go.  The panel is a keeper.
Challenger had the same kind of &quot;faulty breaker&quot; problem that Federal Pacific did.  However unlike FPE, this panel bus design is tip-top, and in fact lives on today in the very popular Eaton BR line. Technically this busing requires a Type C breaker, but if you look at any BR breaker on the market, they are dual-listed Type BR  Type C.
If your main panel is also a Challenger, same deal. The old breakers must go; Eaton BR is a listed substitute.