Turning File Vault on, and instantly upgrading to macOS Big Sur is not a problem?
I manage macbook pros all day everyday, and I absolutely do not recommend upgrading OS's in the middle of encryption.
That said, since it's a new machine with no files on it, and since you enabled it already, chances are that by the time I post this, the encryption process is complete (only if it has been connected to power! otherwise the encryption pauses until it IS connected to a power source).