- Follow this guide to the end (the last stage will appear to have failed)
- Temproot with VISIONary r13 (install the downloaded file as per a normal APK)
- Once you are temprooted, connect via 'adb' and do 'su' (to get root) then '/data/local/flash_image misc /data/local/misc1-2.img'
You should then be able to downgrade in the normal way.
You try this at your own risk... I successfully did this on my device, but of course YMMV.