Previews lost upon upgrade from LR CC 2015 to LR Classic CC

I have a large catalog (~500,000 images) and typically remove the originals from my HD after importing into LR and backing them up to BR disks and an external hard-drive.  I then identify images to work on based on the previews and restore the full images from the BR disks as needed. After upgrading to LR Classic CC the only previews in the catalog were for images that happened to have been restored to HD for my current project.  

After working with support this evening (did a great job, thanks!) the conclusion was that I need to relink to the original files to get the previews in the LR Classic catalog.  There appears to be no way to leverage the previews built previously for LR CC 2015.  Given the large number of original files, this is not a feasible option for me and the support person helped me revert to the 2015 version.

The support person will escalate this appropriately through Adobe channels, but I also wanted to record this here as support for others in my situation.  Of course if there is a way to transfer the old previews to the Classic catalog without restoring the original images I'd love to hear about it.