From when the HD wallets were first introduced back in v0.13:
Newly created wallets will use hierarchical deterministic key generation according to BIP32 (keypath m/0’/0’/k’). Existing wallets will still use traditional key generation.
Backups of HD wallets, regardless of when they have been created, can therefore be used to re-generate all possible private keys, even the ones which haven’t already been generated during the time of the backup. Attention: Encrypting the wallet will create a new seed which requires a new backup!
Wallet dumps (created using the dumpwallet RPC) will contain the deterministic seed. This is expected to allow future versions to import the seed and all associated funds, but this is not yet implemented.
HD key generation for new wallets can be disabled by -usehd=0. Keep in mind that this flag only has affect on newly created wallets. You can’t disable HD key generation once you have created a HD wallet.
There is no distinction between internal (change) and external keys.
HD wallets are incompatible with older versions of Bitcoin Core.
So, no... You can't upgrade your wallet to be HD. You will have to create a new wallet and transfer your coins to an address generated by the new wallet. Also, you don't get given a mnemonic or anything with Core... Basically you just need a single backup of the wallet.dat file and it can regenerate all addresses... So no more worrying about the 100 address pool limit issue and needing to do regular backups etc.
As noted, you can also see the actual seed by using "dumpwallet"
