The only way to make sure that your Bitcoin has the benefits provided by an HD wallet is to send the Bitcoin from your old wallet to your new wallet. Importing the private keys will not be any more useful.
First make sure that Bitcoin Core is shut down. Then make a backup of your wallet.dat file. Move or rename the wallet.dat file so that there is no longer a file named wallet.dat in the Bitcoin Core data directory. Start Bitcoin Core and it will generate a new HD wallet and a corresponding wallet.dat file. Get as many addresses as you need and copy them somewhere. Then stop Bitcoin Core and make a backup of the new wallet.dat file. Move or rename it so that there is no longer a file named wallet.dat in the datadir. Then move or rename your original wallet file back to wallet.dat in the datadir. Start Bitcoin Core again. Send Bitcoin to the addresses that you copied down. Then stop Bitcoin Core, and replace the wallet.dat with the new wallet.dat that was created.
First make sure that Bitcoin Core is shut down. Then make a backup of your wallet.dat file. Move or rename the wallet.dat file so that there is no longer a file named wallet.dat in the Bitcoin Core data directory. Start Bitcoin Core and it will generate a new HD wallet and a corresponding wallet.dat file. Get as many addresses as you need and copy them somewhere. Then stop Bitcoin Core and make a backup of the new wallet.dat file. Move or rename it so that there is no longer a file named wallet.dat in the datadir. Then move or rename your original wallet file back to wallet.dat in the datadir. Start Bitcoin Core again. Send Bitcoin to the addresses that you copied down. Then stop Bitcoin Core, and replace the wallet.dat with the new wallet.dat that was created.
It's clear that im going to take an entire day or 3 in order to make all the transactions, because as we know bitcoin is not that fast anymore. Also it's going to take a bit of fees. This is why i've been delaying the migration besides the fact that im generally lazy.
