Should you need copies of the system icons from Mavericks or earlier, if you still have the system installed on another Mac or external drive, you can find them in the System/Library/CoreServices/CoreType.bundle/Contents/Resources folder (or you could suss them out with Time Machine if you're wise enough to be using it). If you manually apply one of these icons to a Yosemite file or folder by copying and pasting from the Get Info window of one to the other, you can manually revert the icon by selecting it in the Get Info window and clicking Command-Delete - as the copy is pasted on top of the original rather than replacing it. You can probably do the same with YosemiteRevert if it pastes the old icons over the new ones rather than actually replacing the system resources. You will have to examine the Apple Script to see exactly how it works. Copy/Paste is the safest way to do the switch because it does not affect any system resources. Alternatively, if YosemiteRevert does swap out system resources, to be on the safe side you could make a copy of the .../CoreType.bundle/Contents/Resources folder before using YosemiteRevert and put it in a safe place, say the same Contents folder after renaming the Resources folder copy to something like Resources_Original. That is certainly the safest strategy if you don't know how YosemiteRevert operates.
I had already replaced some of the Yosemite icons with the Mavericks versions before I saw YosemiteRevert here on MacUpdate. Which is how I came to know where the icons are stored. If you're not comfortable reading Apple Script, you could make a copy of the Resources folder, invoke YosemiteRevert and then check to see if the icons in the original Resources folder have been changed. If not, then you know YosemiteRevert uses Copy and Paste. The developer could save everyone a lot of trouble and anxiety by doing a better job of explaining his app.