I think you can run the 2 in parallel. Express is a named instance (MyServer\SQLEXPRESS) and you can set the Developer version as Default or as named instance. You shouldn't have any problems after that.

This being said, I think you actually want to get rid of Express. Just use the Uninstall routine for the SQL Server service from Add/Remove Programs. It will subsequently uninstall most of the things that came with it. Then you can install the Developer version