Is there a way to "downgrade" (revert) an HTC Desire HD to stock Froyo from Gingerbread? I don't have any backups or whatever. My phone isn't rooted. It updated to Gingerbread 2.3.3 OTA a couple of days ago, and I hate almost every change that it introduced.

Thanks.
Stock Froyo, I assume you mean Froyo with HTC Sense on top of it.

You would have to root your phone and then load a Froyo ROM.

How to Root HTC Desire

See here for instructions to install custom rom: Modaco Froyo with Sense