Although it would be fine to switch your 60A breaker with a 50A breaker (provided that your wiring supports 60A current - i.e., it was "correct" to start with) and keep the 60A for later re-install (if you ever want switch back to 60A, and remove the 14-50). IIRC, 40A requires 8 gauge, and 6 gauge is either 55 or 60 amps (can't remember, actually). This is copper, BTW.
For 60C wiring methods (NM, UF, LFNC, some LFMC, SER under some circumstances and code revisions), the ampacity of #8 copper is 40A, and of #6 copper is 55A (which means that for a non-receptacle fixed load of 55A or less, you can protect it with the next size up breaker, 60A). For 75C wiring methods (basically anything else), the ampacity of #8 copper is 50A.
