In addition to converting your code to C++ and MFC, you may be planning to port a 16-bit version to 32 bit. If so, you're advised to convert to MFC first.
Porting a 16-bit C application to Win32 is harder and more time-consuming than porting a 16-bit MFC application to Win32. The MFC framework takes care of many of the details, such as message unpacking, that you would otherwise have to do yourself.
Tip For information on 16-to-32-bit MFC porting, see the article "MFC: Porting MFC Applications to 32 Bit" in Programming with the Microsoft Foundation Class Library in Visual C++ 2.0. For Visual C++ 32-Bit Edition, the earlier 32-bit release, see "Microsoft Foundation Class Library Update" in the Class Library Reference.