I accept the master public key would change due to the new seed... but we can already export/import labels now. So presumably if we can import the old wallets addresses (at least those that have had at least 1 or more transactions) then the label import would line up with those old transactions. **Then** enable LabelSync and force an upload.
You might be confused here importing wallet or BTC address to a new wallet will generate a new watch-only wallet without a master public key.
Meaning once you imported these public addresses the label won't sync except the transaction history. The server for labelsync won't recognise it because it doesn't have the Master Public Key.
They use MPK function as wallet_ID without the MPK how the server would determine if the wallet is sending different MPK?
All labels are encrypted recorded on labels.bauerj.eu it retrieve the hash of your MPK to identify if the hash is match to one of their data recorded on their server.
Read more here:
https://bitcoinelectrum.com/how-to-use-the-electrum-label-sync-plugin/I don't know if exporting the labels locally and import it once you created a watch-only wallet would work but I think this one can't force to upload if you import only a public or BTC address like I said it would created a new watch-only wallet unless if you imported master public key.