non-HD WO to HD WO
It seems Electrum supports both deterministic (HD) and non-deterministic watch-only wallets but you can’t simply convert from one type of watch-only wallet to the other type of watch-only wallet.
You can create a new HD wallet and manually import into it the addresses from the old non-HD watch-only wallet. I’m not sure there is any reason to do this.
WO to spendable
If you want to convert a watch-only wallet to one that can spend the money watched by the watch-only wallet, you can’t do that without additional data that isn’t in the watch-only wallet (private keys).
It may help to read the description of Electrum Watch-only wallets at https://bitcoinelectrum.com/creating-a-watch-only-wallet/
A watch-only wallet is a wallet without any secrets in it that could be used to spend bitcoin.
If you have the private keys that are omitted from a watch-only wallet, you can import them into a new wallet following the guide at https://bitcoinelectrum.com/importing-your-private-keys-into-electrum/
