Before you start, I would copy everything off your SD card in to a folder on your desktop.
You will need to create a Goldcard - firstly, you should download
GoldCard Helper - use the app and note down your MMC2 number (this is already reversed)... you will need this later.
Head to
How To Easily Create Gold Card For HTC Desire Without Hex Editing and follow the method to create a GoldCard. The tool is actually for the Desire, not the Deisre HD, but it can be used to create the GoldCard for your phone (it's what I did when I rooted my phone a long time ago). As you already have have the reverse CID, you can put that in to the website that is linked here to get the GoldCard Image as per the instructions in the linked GoldCard tool.
Once you have created the GoldCard, you would need to run the following RUU -
http://db.tt/GivpLPVm (this is a ROM Upgrade Utility - an exe file that you run on your computer to flash stock firmware on to your phone). This version is an older version that is rootable.
Once you have got flashed the above version of the firmware on your phone, you will now be able to root the phone using Visionary and then run the s-off tool (which allows you to change your CID to the HTC one, which I think is HTC-001) - follow
this guide.
Finally, you would need to run the most up to date, non-branded, stock RUU -
http://db.tt/71wHEg87 - this will put the most recent version of the stock HTC firmware on your phone and you should receive updates OTA, without any problems, direct from HTC (no waiting for your carrier to add their apps).