First of all its really easy to learn a new language after you mastered one, especially c++, since it's sort of low level.
Second, all you need is a book and a few hours a week and you'll learn a new language in a month or so. You should be competent in that language by then.
Third, going back to school would be way over kill, it's not that big of a deal to learn a new language especially going from c++ to C# since c# is based on c++ with the c style syntax.
The only other thing that pops up is getting to know .net, but that's not hard due to reference sites like msdn.com.