This process would affect both the compiler and the library. The compiler needs to generate real async-await functions, and the library needs to be changed to use the async methods on Socket.
If this was the default C# library, then the language version required would jump to C# 5, the .NET runtime required would be 4.5, and Mono support would become "beta". I think a better way would be to release a second .NET 4.5 library, with a new language parameter "csharp:5", or a new language "csharp5". I'm more in favor of the former, but it may make the compiler a little hairier to write.