I have recently upgraded from 8.0.1080 to 19.1. When I opened an existing solution I was expecting it to be upgraded automatically however the projects simply have "needs migration". Prior to installing 19.1 I uninstalled my previous plug-in and runtime to start fresh. How do I get my existing solutions/projects to upgrade to 19.1?
It is probably an issue with how you opened the solution. Try opening the solution from Studio; File-->Open Project/Solution. It should prompt for the upgrade then. I also think you can right-click the project in Object Explorer and select reload or a similar option to trigger the load and subsequent upgrade.