For those who still dealing with this problem, I received today my new S9+. I'm upgrading from S7 and I have the same problem with SmartSwitch. Saldy for me, this software sucks when it comes to back up from one device to restore on another. I solve this problem quickly by backing up the messages to my Samsung account on the S7 and restoring to the S9+. Didn't have any problem.
Hope this helps with all of you!