My felony was reduced to a misdemeanor in Calif., can I legally own a gun?

The expungment has nothing to do with it -- relief under Penal Code section 1203.4 does not restore gun rights. However, when a felony is reduced to a misdemeanor it becomes a misdemeanor for all purposes. Since a theft conviction does not have any effect upon gun rights of which I am aware, you should now be able to legally possess a firearm. There is a form available on the California Attorney General's website where you can verify that you are legally permitted to own a firearm. You should check that, but I am reasonably confident that this conviction will not disqualify you from future possession of a firearm.