How would you transition existing external People to SharedChannel?

stefanwey-umb you enforce MFA in exactly the same way you would for guests, through conditional access. If it's already configured then it will work already.

There is no conversion, shared channels need to be created from new. You could copy across files etc. from another channel, but you can't migrate conversations. At the moment it's best to see it as a clean start.

I have been thinking about writing a script to translate the membership of guest in a Team to a Shared Channel (in another Team) and come up with a sort of methodology that can be applied in this case, something for when I get some quiet time.