I guess it depends on how your game has been made. If you have incorporated all the media into the EXE then someone could just download the patch and it would become the real thing. To overcome this, you could have a section on your site where only registered users can download it. Of course, this still means that someone could just leak the patch out, but anyone going to the trouble of searching probably wouldnt have been a potential customer.
If your full game is seperate from the demo then the patch idea wouldnt work because I'd assume your demo has a limited amount of media. This means that updating the EXE file wouldnt necessary let the user play the full version. You could have some sort of checks going to on to make sure that this is the registered verion. For example, in the full version, dummy files are randomly created in differnet folders and when you patch it, you search for these files. If the files exist ,you know that the game is registered. Just an idea :)