The clause at FAR 52.204-18 states "The contractor shall communicate any change to the CAGE code to the contracting officer within 30 days after the change, so that a modification can be issued to update the CAGE code on the contract." The contracting officer then modifies the contract to reflect the new code(s) so it gets updated in SAM. Sometimes there are service- or agency-specific procedures for administrative changes but typically, a simple administrative modification is all that is required to update the CAGE code.