Yep, I realize it's an old skin, but was hoping if I saved it with current Skin Studio it would save in current format, but perhaps I'm wrong.
OS: Win 7 64-bit
Hope this helps without writing a book here. As far as the current format it's the same as it always was. (.wba) The only think that would ever change is if changes were made in SKS itself and then you saved it. The only thing that would change is what you changed. The file extension would still be .wba. If you have noticed every time a new OS came out so did a new version of WindowBlinds and SKS. I'm not the technical person to explain this but basically it's this.
In order to skin the correct areas on the new OS SKS has to be upgraded to include these areas and WindowBlinds has to be changed to skin these areas which will make it work on the current OS. To make the xp skin work on 8.1 you would basically have to make it over again. You would have to add many new images. change text setting, etc everywhere in SKS and more. What you would be changing are many areas that were not needed in each prior version of Windows. There are also many areas in a xp skin that are not used in the current versions of SKS.
So the bottom line is you can make it look better on the win 8.1 you are using but you would need to know exactly where to go to make the correct image, add the images and changes settings.
This is a basic explanation but hopes it helps you to understand the situation.