Upgrade path for iTunes Connect TestFlight external testers after release to App Store?
The pragmatic way to ensure all testers are off the app is to wait the 30 days for the build to expire and have them manually grab the store version.

You could also upload a new beta build instructing them to do the same. That way they get an email from Apple with the news you want to convey to them and a limited duration to get them all off that build.