First of all, I don't know what you mean by CA.  California, or Canada?  Or maybe Confused Alien.  And I wouldn't jump through the hoops that IRS suggests.  I would prepare the 1040NR, mark it "Amended from 1040" at the top, and paper file it with a check for the difference between what was paid and what should have been paid.  Include that computation in the explanation.  But if you want to spend half a day on it, rather than half an hour, it's a slow time of the year anyway.