Yes. There is a birthday rule in California for Medicare supplement/medigap enrollees allowing for a special election period. If you find that the premium for your current supplement plan is lower with another carrier, or if you want to change to different Medicare supplement plan, you can submit an application to the new carrier within the 30 days following your birthday to be effective on the 1st of the following month as a guaranteed issue (no health history underwriting).
Remember that Medicare supplements are defined by Medicare and cannot be changed by a carrier. All carriers can offer the same Plan F or Plan N at different premiums, but the plan benefits will not differ.
However, if you decide to “go down the letters of the alphabet” i.e., from a Plan F to a Plan N, you will not be able to go back up the letters of the alphabet to a Plan F without undergoing health history underwriting.