Wel as far as i know the .NET 4.0 RC and it Release version wont have any controls. it may have any bug fixes it has faced in RC release and nothing more than that will be included.

About the conversion process, it always recommended  to use the original release conversion tool. it will be gr8 and you wont have to face much main in the process. RC conversion always have some backdrops in conversion aspect. i have faced this problem when doing it from 2.0 to 3.5 RC, and 3.5 original version. it had gud enough difference. i suggest if you want to make use of any new .NET 4..0 controls just wait till mid of april and then carry on with it. if you dont want to make use of any new control its better you postpone that, play around with .NET 4.0 controls and then go for conversion with suitable controls.