Replacing non-App Store apps with VPP version

New Contributor II

I'm looking to replace a non-app store installed version of Slack with a version I've set up through the App Store and VPP. The VPP version is already published in Self Service, and is set to Auto-Update but that wouldn't affect any users that had downloaded the app direct from Slack's website right?

How would I set this up to run a report and then have any machines with the non VPP version be replaced with the VPP version?