When I select "Start the new client" under "Sync Client for SharePoint" settings, what is going to happen to users who still work with Groove.exe? Does Sync with Groove still work? Any pitfalls to think about before switching to the new client?
Groove should continue syncing for those who already have sync's running, and would only change over to the OneDrive client if you run the command line switch to change over. I did this with a customer recently - set SharePoint to sync using the new client first, then switched over the desktop client for users and it ran smoothly.