I already tried that one but it seemed to screw up superuser badly. No program was getting root access. According to the developper of Superuser this has something to do with it (Superuser) being made a system app.
Installing via recovery should work. Here's a zip I knocked up. SuperUser 3.0.6
Trouble is you're going to have to reapply this every time you update MIUI. Do you mind me asking why you want to upgrade? What does the new version give you that the old one doesn't?