If it is a non-HD wallet, upgrade it with -upgradewallet (introduced in 0.17+, I believe).
Then use -migratewallet in 0.25 to upgrade to the descriptor wallet.
Sorry, I have a small OT question about this:
I thought all wallets were HD wallets. But I think I misunderstood something. Legacy wallets newly created with Bitcoin Core are hd wallets, but the old ones from 10 years ago are/were not, right?