The PNR will be need to be split-with the person changing to a new PNR.
The original pax will stay with the same seats.
The new PNR- will lose the seats on the return as that pax is not flying with a silver. You could ask as a courtesy.
Outbound seats in Club will stay -make sure to request a "to complete a party" which will advise that the new PNR is traveling with the old PNR.....and the Silver Member.