There are 4 ways to reduce CYA:
1. drain and refill.
2. run with unstabilized chlorine for a couple of years.
3. use melamine -- it's been a mess, but Kem-Tek's chief chemist told me they expect to release a melamine kit with an effective clean up process this year.
4. let your pool go slimey . . . and let the bacteria eat it. Takes 1 month or more, and results are unpredictable. You can end up with ammonia as an end product and that's huge mess.
If you go with #2, it IS possible to run your pool, perfectly successfully with a CYA = 400, but it will take some adaptations. If you have a SWCG (salt water chlorine generator) it will be really easy -- even easier than if the CYA was at normal levels. And, if you have good access to inexpensive liquid chlorine (commercial bleach) that's pretty easy, too. But that stuff is more often available in Florida, than California. You have to make it nearby . . . and California doesn't like companies making much of anything in-state, especially not chemicals.

You WILL need accurate test results -- see the test kit info page in my signature -- and I wouldn't trust the results you got from your pool store. Your actually CYA level may be 100ppm (no big deal) rather than 400pmm.