That's why you need to go read other sections on here... there are quite a few people that have been able to downgrade their 5.10 bootloaders...
on the HTC HD7, all you need is a stock rom image from an earlier rom, a 1GB SD (not microSD) card and a USB Y cable... you copy the main rom file (signed.nbh) to the sd card as SCHUIMG.NBH, put it in a reader on one end of the Y cable, plug the power connector of the Y cable to the PC, and the other end to the phone, and bring the phone up in USB loader mode (not SPL mode, power on, volume down and camera will get you to USB loader mode) from there the phone will see the image, and may reboot up to 6 times as it loads it in. when it is done, you will have the lower version ROM complete with it's lower version SPL. you don't need the CID of the SD card, or any gold card generator. Just use a stock rom for your region (IE; asian rom if thats what your phone came with stock etc...).
I have done it 4 seperate times on my phone to make sure it works...