25 Pa. C.S. § 1503
By the deadline for registration, a registered elector who desires to change the enrollment of political designation or who, although registered, has not previously enrolled as a member of a party may appear before a commissioner, registrar or clerk or may submit an application by mail under section 1324 (relating to application by mail) and state in a signed writing the political party in which the registered elector desires to be enrolled. If the signature of the elector is verified by comparison with the registered elector's signature as it appears on file with the commission, the commissioner, registrar or clerk shall make the change in its registration records. If supported by other evidence of identity, a mark may be made in lieu of a signature by a registered elector who is unable to write. The mark must be made in the presence of a witness who must sign the registration application.
25 Pa.C.S. § 1503