Just had the same problem when migrating assets to a new project with C++ parent classes.
After failed with several solutions (redirecting, etc.), I duplicated the broken assets in my old project, reparented the assets with a parent class existing in the new project (in my case, the broken parent class was inherited from the default UserWidget class, so I reparented to UserWidget), then migrated the duplicated assets to the new project, open Blueprint Editor in new project and fixed all the errors. In this way, I kept most of the Blueprint scripting and fixed the problem with a relatively lower cost.