From first posting ...
"long since discovered (after many a wasted hour and $) that migrating was not going to be an option and total rebuild would be required."
If one thinks about it, you've already answered your own question - total rebuild.
What seems to be hanging you up is a way to facilitate the "total rebuild" with as little pain/work by teachers.
You've not shared what didn't work in the migration. You've not shared number of courses involved.
Teachers will have to learn the new system ... of that there is no doubt. OR did they already with the migration attempt?
So there will be some 'pain' ... some will grumble (can't please everyone), some will try to explore ways to do it quickly (let them ... they may discover something they could share with all other faculty). Can you identify faculty members that seem to have the ear of other faculty .... ie, your power users - the ones who are most in the know? What say they?
'spirit of sharing', Ken