I can only say that when I downgraded my Nucleus from 2.0 to 1.8 legacy the downgrade would not work (after many tries) until I left the remote connected.
The Roon migration FAQ states:
You will need to install the Roon 1.8 legacy app on a Roon Remote/iOS device first before being able to take the downgrade.
I assume that in order for the Roon server to know that you have done that the remote has to be connected as part of the downgrade process. At least, that’s what finally worked for me.
Stephen