In this Andrew Chow blog post on what's coming to the Bitcoin Core wallet in 0.21 he states:
It is, and will be, possible to take an existing Legacy Wallet, and make a Descriptor Wallet which exactly matches it. There will be tooling that allows users to do this. Even after the removal of Legacy Wallets and Berkeley DB, it will be possible to migrate Legacy Wallets to Descriptor Wallets through the use of a minimally dependent migration tool.
How will this migration tool work? How can the descriptor wallet obtain the descriptor from the legacy wallet given that the legacy wallet doesn't know what a descriptor is?