For the moment it is not possible to migrate automatically the old widget to the Glance version.
A ticket is open at Google : https://issuetracker.google.com/issues/294137086
In my case, the old widget is deleted when I install a new version of the app with the Glance. The only way that I find, is to store the data from the old widget (Data Base, Share pref...). And when the user wants to create the new widget (Glance version), I get the data from the old widget and I display them directly after the creation.