It will downgrade everything even the HBoot using a HTC signed ROM. I've downgraded RUU_Saga_Telstra_WWE_1.36.841.3 with HBoot 0.98.0002 to Hboot 0.98.0000 of RUU_Saga_HTC_Thailand_1.35.1113.2.
First you need below tools and applications. I will not explain everything since I'm assuming you already know how to use it and make it.
1. ADB tool to access you device thru shell.
2. ADB driver - you may install HTC Sync since it has ADB driver in it.
3. HEX Editor - I used HxD.
4. Spare micro SD with Goldcard.
5. Card reader to make your life easier.
6. Update.zip ROM you will use to downgrade(rename it to PG88IMG.zip).
6. GingerBreak-v1.20.apk to temp root our device.
Step 1: Copy GingerBreak-v1.20.apk to your spare micro SD and insert it into your phone.
Step 2: Enable USB debugging in your device and connect it to your PC(Charge only). Make sure the drivers are installed properly. If not, install HTC Sync.
Step 3: Install and run GingerBreak-v1.20.apk. It will force close other apps(this is normal just close it). The gingerbreak application will promp that something goes wrong with the rooting(can't remember the actual spiel) but actually we already have our temp root.
Step 4: Run you ADB tool and issue command su to have root access. You can now see in your device that Superuser app is prompting you to allow the ADB root access. Accept it.
Step 5: On the # prompt, issue command dd if=/dev/block/mmcblk0p17 of=/mnt/sdcard/mmcblk0p17.img (to copy mmcblk0p17 to your SDcard). Power off your device and copy mmcblk0p17.img to your PC.(You can also use the command shell to copy it into your PC if you know how to do it).
Step 6: Open mmcblk0p17.img using your Hex editor. On the 11th line(I think), modify the current version to 1.28.401.1(since this is the lowest version I know). Save it and copy back to SDcard. Insert the sdcard and turn your phone on.
Step 7: Run GingerBreak-v1.20.apk again and follow step 3 to 4.
Step 8: On the # prompt, issue command dd if=/mnt/sdcard/mmcblk0p17.img of=/dev/block/mmcblk0p17 (to copy back mmcblk0p17.img to your phone). Do this as quickly as possible since the temp root access sometimes loose its effect.
Step 9: Power off your device and remove your SDcard. Using your card reader, delete everything(not format) in your microSD(with Goldcard) and paste your PG88IMG.zip.
Step 10: Hold volume down + power to boot to recovery and the phone will do the installation itself. Wait until you have your downgraded ROM.
This is how I do it. Hope you won't encounter any problem with this procedure. Good luck!
I want to give thanks to all the XDA members for the knowledge I acquired for this procedure and to the developer of Gingerbreak.
You may also check sonikz procedure on post #4. I think his procedure is faster. You may use which one is easier for you to follow.