The new testflight platform included with iTunes Connect is different from the old platform. There are no methods to automatically move your assembly and users to the new TestFlight.
The new TestFlight manages two types of testers: internal and external. These two types are explained independently, and you need to configure each user that you have on the old platform. You will configure users by mail, up to 1000.
The assembly you must verify must be loaded using the classic application submission method. At this point, after a couple of minutes, you can enable the assembly for testing. Apple must review the assembly before turning it on; in a couple of hours (late, 1 day) you will have a new build for distribution.
Finally, when the build is available, you can distribute it through the iTunes Connect pre-loading panel: users will receive mail with a link to automatically add the application to the device. No need for UUID specification.
Check the documentation at the following link: TestFlight
Hope this can help you.