When union contracts expire and there is no new contract, can you continue under the existing contract indefinitely?

Not sure if there is a general answer or if it depends on all sorts of legal details, but I was wondering if we don’t negotiate a new contract, how protected are our current benefits under the previous contract? We’re not going to get any new cost of living increases or anything, but can they take away or unilaterally change health care or other benefits without an agreed-upon new contract? (in California)