hi,
at the moment rbutil do just the same as when you install a build manually, so no removing of files.
But it could do it. Rbutil already has a Uninstall method to remove files selectivly.
So you could choose Uninstall rockbox, and select rockbox base to remove the old build. ( only files of the rockbox.zip installed by rbutil are removed, nothing else.)
If you or someone else have time and coding skills, it wouldnt be to hard to combine this (uninstall old rockbox base, install new rockbox zip) and map it to a button.