Need to re-enroll 200-250 iPads worldwide, help?
You can only have 1 MDM profile installed at a time, so it will not allow you to re-enroll the devices if they are already enrolled (even if the original server is now defunct)

I cannot recall if you can remove the MDM profile from a supervised device, but I believe that you can.

The easiest way would be to probably send a link that can be opened in safari to re-enroll the devices as you mentioned above. You can always take the profile and host it at a different URI so no user account is needed, therefore you wouldn't need to worry about access at all.

Hope this helps a little.

Mike