Desire HD, G2, Desire Z, and MyTouch 4G Froyo Downgrade
The trend over the last month or so on XDA has been getting a number of devices downgraded so they can get to a bootloader that can still be unlocked with S-Off. Those devices are, of course, almost all HTC devices ever since the release of HTCDev, which unlocks the bootloader while maintaining S-On. In the march of obtaining S-Off, a few devices have already had a contingency plan in place to get users down to a software version that can be properly hacked.
XDA Forum Member Setherio has been maintaining and updating a thread for nearly a year now that allows the HTC G2 and Desire Z, the HTC MyTouch 4G and the HTC Desire HD to be downgraded to Froyo in order to then update to a rooted Gingerbread ROM. The process is quite a long one, but that is to be expected when it serves as a guide for multiple devices. The process runs users through gaining temporary root, the entire downgrading procedure—including Fastboot downgrades—and then reminding users to not apply any large OTA updates to avoid being thrown back to unrooted Gingerbread and forcing them to start the process all over again.
As root methods for newer iterations of Android have been getting more and more difficult for HTC devices, any method that can bring full root should be looked at. As usual, be extremely careful when completing the steps, and make sure the steps you are following are indeed for your device.
For much more information, a plethora of download links, and even some troubleshooting help, head over to the original thread and let the downgrades continue!