If you live in California ignore the registration & go vote in person.  That works for the general election (you need to be a registered voter, but everyone gets the same ballot and can vote for whoever they want regardless of party registration) but in this thread we’re talking about the primary in March. Walk-in or mail-in, you won’t be allowed to vote in the Republican presidential primary unless you’re a registered Republican.  That’s a party rule, nothing to do with the state.