Subject to Sections 21223 and 21224, a person who has retired from state civil service may be employed temporarily in a civil service position at any time following retirement, provided that the position is either:
(a) In the class in which the person had permanent or probationary status or a career executive assignment appointment at the time of retirement.
(b) In another class to which the person could have been permanently transferred, reinstated, or demoted at the time of retirement.