When changing the Server in the aggregation editor, it's likely that the developer would want to continue working with a database and collection of the same name (i.e. when running an aggregation against a development environment first and running it against production afterwards).

However, when the server is changed using the server dropdown in the aggregation editor, the first database on the server and the first collection of the database are selected.

I'd propose selecting a database with the same name as the one that was selected before, if there is one. The same should work for the selected collection.