When I killed my very old raspiblitz node, I put up at notice on Amboss, 1ml, etc just stating that it was being retired on a certain date. Once that day hit, I waited until fees would be low and closed all the channels. Then once it was offline, I updated the details again. There were ways to merge over your current node to Start9, I just didn't want to. My current one is running on a Start9 now.