Legal Question in Real Estate Law in California

I don't know. Got your contract handy?

The rights of the borrower and lender are determined by the black-and-white terms of the loan agreement. Either the lender can raise the payment, or the total amount due, or the interest rate, or whatever, or it cannot. The word "fixed" can mean several things, or perhaps the loan agreement calls for it to be "fixed" only so long as certain factors exist. There is no absolute, correct in all situations answer except to read, understand and accept the terms of the governing agreement.

"Fixed"? No, other than for taxes withheld that increase over time. Consult your loan docs to make sure it is 'fixed', not 'fixed for xx years' with adjustable thereafter.