For this the android sdk is to be installed, providing the 'adb' binary for communication with the phone
The old ROM is then placed on the SD-card. Starting the downgrade failed; a checksum was found to be incorrect by the updater.
To remedy a 'golden card' had to be created
For this the SD-cards serial number is read out. A command like 'adb shell cat /sys/class/mmc_host/mmc1/mmc1:*/cid' is to be used. Yet this didnt bring up anything useful on the HTC desire HD, but it did for my HTC desire. The SD-card inside the HTC desire was prepared to be used as 'golden card' and the downgrade performed with that card.
This serial numbers order is reversed
This reversed number is then embedded into a bootsector, there are websited doing this
The sector is written onto the already partitioned SD-card, after having the mobile offering the SD-card as storage: 'cat goldendisc.img >/dev/sdb' - if the mobiles SD-card appears as /dev/sdb.
With this the downgrade succeeds
Then a recovery software can be installed like ClockworkMod
And custom android versions can be used
there is no usable cyanogenmod 9 for the HTC Desire HD yet