SRJ should stay in the cast list, but not in the front page popular cast, if he's no longer in the show. Right now, the active 7 should be Shiva, Parvati, Rama, Sita, Lakshman, Hanuman & Ravan.
SRJ - or whoever plays Vishnu - can return whenever the Ramayan track is over - as it is, at that point, a whole cast overhaul will be needed. Particularly in this serial, where Vishnu & Rama never appear simultaneously.