There are two proxies of the same model [one active, one standby] in the same failover group, and the active will be upgraded to a version of SGOS on a different release path (6.5 v. 6.7, for instance).
It is most recommended to run the same SGOS version whenever possible, but there generally should not be an issue. Consult the appropriate Release Notes for the later version for any possibly relevant changes to multicast, if any.