M-files Desktop Client - button to toggle between old and new versions is missing.

Your administrator may have hidden the toggle. Technical details in section 3 of Quick Guide - New User Experience for M-Files Desktop.

Another possible reason is a change in your desktop client’s connection protocol. The new M-Files Desktop requires a gRPC connection to the server. This setting normally doesn’t change on its own, but administrators may sometimes update vault connection configurations.

Thanks so much for your guidance Joonas.  You are a lifesaver.  I had to go to Admin and go to Configurations \ Advanced Vault Settings \ Configuration \ Client \ Web and New Desktop \ M-Files Desktop Settings.  I appreciate your help!