You don't need to downgrade hboot to install endy.
- Create goldcard (with other device if necessary) and install whatever ruu you can.
- Update ota to the latest version available
- Setup device to your preferences
- get your device's bootloader ***UNLOCKED*** trough htcdev
- install (flash via fastboot) REVOLUTION, or 4EXT recovery img
- perform nand backup via recovery
- install your favorite rom (detailed instructions here
ps: if you didn't understood latest step in directions how to install rom in linked post, you extract boot.img file FROM ZIP OF ROM YOU ARE INSTALLING.
Similar stuff goes for returning to stock. It's not enough simply to restore trough recovery. You must flash boot_signed.img after you performed restore process.
pss: if you think there are some excess steps in my directions feel free to skip, but only if you are 100% sure you know what you're doing...
You can also root device installing SU via recovery
And one more thing... While playing with ruus, if anything goes wrong there are pretty decent chances that you will be forced to pull out battery from your device. Make your best effort to avoid this, and if you really must, than wait at least 30sec before putting battery back in your phone.