Changing the extension usually works. For example, changing the extension of a text file from txt to rtf changes it to a rich text document automatically for me, and asks me if I'm sure I want to change the extension before it does so. Something similar happened when I tried to change the extension of my texture from gif to png, as it asked me if I was sure and changed the extension in the name to .png, but unknownst to me, it was still a gif image in the background. It wasn't like I was just changing the file's name from somegifimage to somepngimage, I was under the impression that Windows was going to auto convert to for me, given that it warned me and I've converted files in that same manner previously.