In short: You may upgrade your VS2010 project to VS2012, however Windows forms applications will not explicitly upgraded.
Option #1: If you still want to keep the WinForm project and not convert it to the WPF application then you have to use WF4.5 features through services.
Option #2: Although Microsoft removed the option to create a C++/CLI Windows Forms application, the template files are still installed. The only thing missing seems to be the .vsz files and a registration in the vcNET.vcdir file. I have recreated these files and put them up for download here.
Install the files and you should be able to create WinForm apps in C++/CLI again.
Option #3: You may still upgrade your projects separately according to this post - FREE: Visual Studio Express 2012 for Windows Desktop.
My advice before upgrading to .NET4.5, would be check all your third party components, and make sure that they have updates for upgraded version of framework.