Doubt adding F5 in sync-group

I need to replace an LTM-GTM cluster. I will replace the standby node first. The sync-group are 4 devices. My question is about adding it to the GTM sync group. Should I run only the gtm_add command, or do I also need to run the bigip_add command? I'm not sure whether joining a sync group requires both commands or just one of them. I'd also like to know where each command should be executed. I understand that gtm_add is run on the new node being added, but is bigip_add also run on the new node? how can i delete the old nodes for the sync-group when its donde the change?