We have a very common customer scenario where the customer decides to upgrade to a new version (in this case let's say SQL Server 2016). They run upgrade advisor and note that there are a whole lot of problems. So they don't deal with them. They upgrade to 2016 but leave their db_compat level at a lower level with the intention of dealing with the problems later.
But when they go to run the upgrade advisor later, to find the issues that they need to resolve before changing db_compat level to 2016, it tells them that they can no longer use it because their server is already at 2016. Yet they also cannot now reattach their databases to an earlier version server either.
How do they now find the issues that need to be resolved?