You are not alone.
The Migration Tool doesn't work worth a tinkers.
You have until August 2 to make the change on your Good Til Cancelled listings.
I'd suggest doing 10 or 20 a day every day for the next two months. Boooooring but necessary.
Sellers with PCs who use TurboLister have reported some success using that to make the changes. I'm on Mac and TL is not available.