Legal Question in Criminal Law in California

You're asking if you (or somebody) can take back their waiver of conduct or custody credits after it's already been done?

If it was part of a plea deal, the person would have to make a motion to withdraw their plea. If granted (and I'm not sure it would be without further details like why they want to try and undo this), all original charges would be reinstated and any plea deals may be worse.

Whoever the person you're talking about is needs to discuss all this with their attorney.