The sdk itself doesn''t have debug/retail versions - it''s just the help files etc. It''s the DirectX libraries that can choose between and you should have no problem switching between the two modes. In control panel there should be a DirectX entry now with an option to select which option you want (I think this only appears if you install the debug version initially). You can reinstall back and forth as much as you want.

For developing you probably want to use the debug libaries as they tell you stuff like why things aren''t working. However they aren''t always a good indivation of how fast your program would be in the real world so you should swap to retial drivers every now and then to see how fast it really is.

There are also some handy options in the DirectX control panel thing, like how much information DirectX gives you while running and so on.

Hope this helps.