Can a California tenant change the locks without landlord permission?

Generally no, unless the lease permits it or the tenant has a statutory right (such as a domestic violence survivor protection in some California statutes). In a typical California tenancy, changing the locks without notice and without giving the landlord a key is a breach of the implied right of access. The landlord retains a limited right of entry for inspections, repairs, showings, and emergencies. A tenant who unilaterally changes the locks can be served a cure-or-quit notice, and in repeat cases, eviction. The proper sequence: ask the landlord in writing, propose to share the new key, get written approval, then change. If the tenant has a safety concern (domestic violence, harassment), most California jurisdictions allow lock changes after providing notice and (where required) documentation, with a duty to give the landlord a copy of the new key within a short window.

Source: California landlord-tenant code; common-law right of entry