Just the GUI parts is pretty easy.
The hard part is making all of the guts of the application(s) work cleanly with the GUI, update other windows properly, not have unexpected side-effects, and sp on, especially when the existing guts were not written with sucg a GUI in mind.
It takes a lot of thought, planning and preparation. It can feel like one has a large, challenging mountain to climb with many risks of falls. Doing a major redesign can be scary. In my experience it usually goes far better than one fears, but not always. How confident would one be if the redesign estimate was five man-years of work? Or ten? Or thirty?
One of the issues with using 'builders' is that sometimes the builders don't keep up with progress and are superceded in ways that aren't always easy to accomodate. I've had a superb GUI buillder just not be updated for years due to low uptake because the underlying programming language went out of fashion. I now have a GUI product that I can't update without a total rewrite from the ground up! And I cannot justify doing that.