Rats. I’ve got my old phone sitting waiting to go to my daughter but would love to sync across the messages first… now I don’t need them… but if I can keep them it would be nice.
I have an app somewhere (who knows if it still works on High Sierra) that can copy off all the messages and print them as a PDF or something like that. I’m sure there are other options too.
Plan B would be to do a backup, save it off for later and give her the phone… Then when the appropriate update comes, back it up again for her, restore my old image, sync message content the wipe it and restore her backup…
Plan C. Just move on