Originally Posted by some_douchebag
if HTC allows downgrading the bootloader
That depends on what you mean by "allow". You cannot downgrade the bootloader by any "official" means. You will fail version check when you try to install, as you can "normally" only flash a equal or great version number hboot.
But if you are s-off, version check is bypassed, and any hboot version can be flashed.
Similar to a previous response, my advice would be to unlock the bootloader (required if you want to flash custom ROMs anyway) and S-off the phone soon after you get it. Neither of those can be changed by any OTA.
Also, accepting OTA is not recommended on a modded phone, anyway. Whatever came in the update will usually get posted in a stock rooted form in the Development forum, and/or incorporated into custom ROMs, often within days of the OTA rolling out. And OTA will not install on a modded phone (stock recovery needs to be present). OTAs also may plug existing s-off or other exploit; preventing you from doing them if you haven't already (but as mentioned, can't make the phone s-on again). Moral of the story, I strongly recommend against OTA on any modded phone.
Some of these terms/concepts are HTC specific, so may be a bit confusing for you. I would suggest reading up on these forums, if you aren't familiar with s-off, HTC's bootloader unlock process, etc.