Probuilder meshes no normals and not visible

I've got an older (Unity 2018.3.0) project that I want to upgrade to 2019.4.10.
After converting I noted that the old ProCore folder is empty, so no old Probuilder files are present.
All the geometry looks fine. The PB objects of course complain about missing scripts. The meshes are all pb-meshes and so not visible in the project folders. See the screen shot of inspector of one object just after the import into Unity 2019.4.10.
If I install the latest PB version and let PB upgrade everything, a large part of my meshes is gone. There is still a reference in the Inspector to a pb-mesh but the meshes are not visible in the Scene/Game view. The Inspector reports there are no normals on the mesh (they were there before the PB upgrade).
The Export option doesn't work because of the missing Normals. I tried it twice (restore the old project, import in Unity 2019.4.10, install PB from package manager, PB convert and recover objects). Gone.
Any ideas how to either get those meshes out of PB when I got the project in 2019.4 or how to upgrade PB properly to keep the meshes as they should be?

Cheers!