File Vault 2020

It takes far less than minute to change that setting.

But to actually implement the encryption/decryption you asked for, every file on your drive must be read in one state, and written back in the other. That means it takes ALL afternoon to complete, provided your Mac does not go to sleep during the process.

You need to wait until it finishes the conversion. Your Mac will continue to operate with a "split" disk, partially-encrypted/partially-decrypted until it finishes. Only then can you change the state again.

Your only other recourse is to cancel it altogether and ERASE your drive re-install MacOS from scratch or from your backup.

It takes just as much time to de-encrypt as it does to encrypt. You might as well let it finish.

FV is an all or nothing option since a partially encrypted volume might as well not be encrypted at all.