Das scheint die Lösung zu sein... Hab das aus nem XDA Forum kopiert. Wenn ich jetzt noch Englisch könnte ;) Falls es jemand wenigstens Stichwortartig übersetzen kann... Wäre nett

And guys, you don't need to complicate that much with all that flashing back and forth. It works with any custom ROM rooted or not, you just need to make the right approach. You don't even need to remove the battery. The only requirements are account on HTCdev, bootloader RUU and operational adb/fastboot environment. And NO, you will not loose the root.
So here are the steps:
1. do a nandroid (at least data partition)
2. download the bootloader RUU from HTCdev and run it
3. search for rom.zip in your profile directory (it was extracted from RUU after you had run it)
4. connect the phone to your computer
5. copy that rom.zip to phone's sdcard renaming it to LEGEIMG.zip (mind the case)
6. reboot to bootloader from command prompt (adb reboot bootloader)
7. current bootloader should find the LEGEIMG.zip and ask you to upgrade bootloader
8. after upgrade is done switch to fastboot (use up/down and power buttons)
9. reboot bootloader from command prompt (fastboot reboot-bootloader)
10. at this point you end up in new bootloader which is still locked
11. follow the instructions on HTCdev from Step 8 to unlock
After that you need to boot into recovery, restore data partition and you're back in the saddle.