Bitcoin Core is a determinstic wallet, by default. If you create a new wallet now, it would be deterministic. I'm can't remember if every wallet would be converted to HD wallets off the top of my head.
Bitcoin Core has been a deterministic wallet since 0.13.0 (
https://bitcoin.org/en/release/v0.13.0)
And no, older wallet files are not (and can not be) converted into HD wallets. If your wallet doesn't have the HD icon in the bottom right, it's a deterministic wallet file. If the HD icon has a cross through it, it's an older non-HD wallet file.
The reason a lot of folks don't realise it is deterministic, is because of the fallacy that the "12/24 words" are "The Seed"... They're not, the words are
mnemonics... In other words, just a way to encode the seed (which is actually just a very large number) as a set of words that are easier to transcribe without errors... Anyway, because Bitcoin Core doesn't give you "12/24 words", people think it doesn't have a seed.
Bitcoin Core (since 0.13) has a "seed", it just hides it from the user in the wallet.dat file... And the backup methodology is to make backups of the wallet file, thus backing up the seed... As opposed to writing down a mnemonic that enables one to restore the seed and recreate the wallet.