Garmin Express recognises my various devices as it did before the roll-out of Connect modern and updates them. There was a recent update to Express and since then the two Edge 500s in the family are now showing greyed out and with a ? in the device selection area (top left corner).
Our Forerunner is fine and will sync with Connect 'modern' while the Edges won't.
The Forerunner has the option to assign an account to it but the Edges don't (despite having been registered to us many years ago).
Strangely the Forerunner is registered to my wife and to her Connect account but shows up in my Connect account as a device....
*FIXED* I deleted my Edge from Express and re-installed it and all is well :D
As for the Forerunner, I just deleted from my Connect account.