Depending on the SDL Tridion release from which you upgrade, you may need to recompile or change your custom GUI extensions.
Migrating custom Dashboard screens If you added custom screens to the Dashboard, you must recreate these screens in the new user interface, because the Dashboard no longer exists as of SDL Tridion 2013 SP1.
Updating DLL references for your GUI extensions If you are upgrading from a version older than SDL Tridion 2013, and if you built a GUI extension for SDL Tridion, recompile (and possibly modify) your code.