The difficulty level of moving the code from Windows to Linux depends on the complexity of the code. If it has a lot of Windows GUI, then it can become a pain in the ...
I am not aware of any free tool that can guarantee a 100% perfect result - there is always some amount of fiddling the source code.
Instead of moving towards KDE-coding you could also consider transferring to some toolkit like FLTK, FoxToolkit or wxWindows. These kits provide code wrappers for GUI stuff that can compile on both Windows, Linux and even Macs. You still have to convert your code, but if that's done, then you will have to maintain only one code branch (hopefully). That should make it easier in the future.
Of course, KDE also runs on Windows, but I don't know anyone who bought QT's Windows version just to build software on that platform as well (obviously, I do not know all persons on this planet :-)