I normally don't use the new spa features, but my understanding is that the pedicure will overrun the CAS nails for a limited time only. I recall back when Spa was refreshed, simmers reported that if doing both pedicure and manicure the first might go away while the sim is still at the Spa, but I think that was a glitch that might have been addressed in a patch, or being caused by CC. Any such treatment should of course last long enough to at least use it for an event the same night. Anything else would be pointless.
I mostly use CC for nails but I guess it's time to pay a visit to the Spa, perhaps for my Spa Ladies club ;)