For an already encrypted machine, I'd suggest the fastest way forward would be:
- Backup all data off the machine
- Change boot options of the machine to UEFI
- Boot to Windows media and install afresh
- Install SEE and encrypt
- Restore data
Doing so this way skips the decryption process, speeding things up (plus you get a fresh build out of it).
To be fair, I don't believe there's even an official process from MS for going from BOIS to UEFI on a BitLocker Encrypted machine that doesn't also involve reinstalling Windows, so it's expected that SEE must follow a similar path.