the change to Samsung account does NOT have to go smoothly or quickly. I got the banner about 7-8 weeks ago, I put it off for about a week, then started it, then it said “Updating your account / Please wait… / It usually takes few seconds to complete the update. If the update takes longer, we will notify you once it completes.”
So I waited. After about 3-4 days of seeing this message and not being able to use the phone app on that phone, it looked tits-up so I called support - I forget the specific issue, but it took about 40 minute of investigation and testing. I do have custom smartapps and device handlers, but I don’t remember if they were involved. This problem did not affect different phones, dashboard or Echo interfaces.
So yeah don’t do it on Friday if your phone is your main interface.
AHH YES I THINK the main fix in my case, was to blow away the phone Settings/ CloudAndAccounts/ Accounts/ myusername(SmartThings) . But the Support Guy may have re-initiated the phone account migration, I don’t know what all he was doing.