It should be possible by following the guide at modaco.com. It takes you from 1.6 to 2.2 and has a process to go back.
If you can't find the links let me know and I will dig them up. I used that guide on my first streak. Got to froyo then back to donut when I had to send it back.
Sent from my Dell Streak
Lumia 920 // Xperia Z // Xperia TL // Xperia ion // Xperia P // iPad 3
Sony Smartwatch // Sony Smart Wireless Headset pro // Sony LiveDock™ // Xperia ion Smartdock
iPhone 1st Gen/3GS/4S // Nexus One // Dell Streak // Atrix 4g // Xperia Play // Lumia 900 // Xperia S // iPad 1
Click here for my Xperia firmware downloads.
Thanks button if I helped.