sethdseed ( "newkeypool" "seed" )

Set or generate a new HD wallet seed. Non-HD wallets will not be upgraded to being a HD wallet. Wallets that are already
HD will have a new HD seed set so that new keys added to the keypool will be derived from this new seed.

Note that you will need to MAKE A NEW BACKUP of your wallet after setting the HD wallet seed.
Don't trust. Verify. Docs extracted from src/wallet/rpcwallet.cpp#L4354-L4369.