will my FEIN change if I switch my california LLC to a NON PROFIT organization?
I doubt it. As long as the organization itself continues to exist then the mere act of applying for and receiving permission to act as a 501(c)* organization and solicit contributions shouldn't require a new FEIN, if the entity doesn't change.
As far as I know, once an entity acquires a FEIN, the FEIN never changes. It's sort of like your social security number - it doesn't change even if you change your name.
I could be wrong, though. I haven't practiced tax work for a LONG time and so things may have changed since I last looked into such issues.
When you apply for non-profit status with the IRS, you must enter your existing EIN. Applying for an EIN and applying for recognition of non-profit status are two separate events, so you should be able to keep your existing number. As always, be sure to call the IRS for clarification of their rules.
Also, I've changed my EIN, and it was not an issue. You use the new EIN at tax time and update your vendors and bank with the new number.