Follow these instructions very carefully. Make sure you backup.

• Start with Bitcoin Core stopped
• Move your wallet.dat file out of the Bitcoin datadir
• Start Bitcoin Core. It should create a new HD wallet
• Get an address from the wallet. Copy it somewhere safe.
• Stop Bitcoin Core. Backup the new wallet file in the Bitcoin datadir
• Move that wallet.dat file out of the datadir to a safe location
• Move your old wallet.dat file back into the datadir
• Start Bitcoin Core
• Send all of your Bitcoin to the address you copied down earlier. Wait for a few confirmations
• Stop Bitcoin Core
• Move your new wallet.dat file back into the datadir
• Start Bitcoin Core. You should see the Bitcoin in your wallet now.