While trying to do the migration, I get the screen below which fails almost straight away! it gives me zero clue as to what the problem actually is… Ive also looked at the web logs and nothing, its like it doesnt even try. I opened to case 3 weeks ago and nothing back from SmartThings despite chasing 3 times.
You can manually migrate items over:
If you have STH with custom rules… you can install STHM in the new app and create automations/scenes for any custom rules.
If you have any routines… you can create new automations/scenes in the new app.
If you have smart lock guest access, you can install the version in the new app.
It will be more work on your part but it can be accomplished if you don’t want to wait for ST support 
Oh, and set up presence sensor for the new app if you use it. Go to menu > settings > use location for this phone and enable it.
many thank, that what I ended up doing… that said my old app still works and can even add virtual switches to it and they show up in ide.
bit a pain re-creating a load of scenes and my sonos now seems 50% Broken…