[HOW TO SAFELY FLASH THE NEW ROM AND BE ABLE TO REVERT BACK]
Currently, the new ROM (1.72.405.3) has a new HBOOT (so you can't install ENG HBOOT S-OFF) and is difficult to root (not as simple as it is with previous ROMs). However, it also has a better radio, and a faster ROM with various bugfixes. This ROM is also Android Froyo v2.21. If you want to try out the new ROM, whilst maintaining the ability to 'revert' back to your older, hackable state then follow the below instructions carefully. Also, do not worry about extracting system.img files and whatnot.
*It is recommended to do a NANDROID BACKUP first
*All RUUs can be obtained from www.xdafil.es/DesireHD/RUU/
*If you don't do steps 1-3 properly and flash the new ROM, you will not be able to revert back
*Radio S-OFF and SuperCID can be obtained by following instructions in this thread: http://forum.xda-developers.com/showthread.php?t=857537
*Tutorial for flashing RUUs (needed for steps 4 & 8): http://forum.xda-developers.com/showthread.php?t=841890
Install Radio S-OFF (NOT
ENG HBOOT S-OFF) and SuperCID
Go to bootloader, then System Info. (If System Info is not there, but you have SHIP S-OFF skip step 3 and proceed to step 4)
Make sure CID is 11111111 *AND* the bootloader shows S-OFF. If not, repeat step 1
You can now flash the new ROM (1.72.405.3)
This will remove ENG HBOOT (if you had it) and ClockworkMod
If you did steps 1-3 correctly, your Desire HD should still be S-OFF. Go to bootloader and check. If not, you did not do steps 1-3 correctly and will have to wait for the new HBOOT to be cracked.
When you are done with the new ROM and want to go back, follow the instructions below:
Download the HTC RUU 1.32.405.6 of your choice (Since you are SuperCID you can flash any RUU)
Flash it to your Desire HD
Root your ROM using Visionary, and install ROM Manager and flash ClockworkMod. You can optionally install ENG HBOOT S-OFF now if you like.
If you made a NANDROID Backup, you can restore this now from ClockworkMod