If you are using CRDT, it depends. You can restart the old peers (rather than the new) with the bootstrap argument or the modified peerstore. If the peers are on the same local network they may be able to auto-discover eachothers via mDNS without having to do anything else by just starting them. Or the new peers may simply discover each others as long as they connect to a peer that is already in the cluster via DHT routing.
My recommendation is to modify the peerstore file, as that is what makes them open connections to places and gives them first-hand multiaddress information for other peers.