From what I understand if you are using Unrevoked Forever all you need to do is find the radio you want, rename the zip file to "PB31IMG.zip", boot into HBOOT (not recovery) and after it prompts you to install, push up to install.
This will downgrade you back to the original radio. This is just on what I read, as I have the 2.15 radio, and haven't downgraded it before. If you go to the 2.15 thread you can see the same instructions, plus the original radio in zip file.
Here is the
thread