After being told for a long time by FetchTV that you can't downgrade your firmware once you've upgraded it (not good for people testing beta firmware out!), it turns out that there is a facility to do this.
Download the older firmware you want to downgrade to and put its unpacked files (autorun.html and upgrade.bin) at the top level of a FAT32-formatted USB stick as usual. Insert the stick into one of the 8320HD's two USB ports (go to Menu -> Media Centre -> USB and select autorun.html if nothing happens) and it should tell you that the older version isn't suitable. Press and hold the Info button on the remote and it will apparently force a firmware downgrade.
However, the snags with downgrades are 1) finding the appropriate old firmware zip file in the first place and 2) whilst this might improve the behaviour of some later firmware bugs (e.g. HD exports, remote control repeating etc.), it will also bring back nasty old bugs and crashes galore. Early firmware releases were incredibly crashy for me and I wouldn't really want to go back to any of those.
Thanks to MaffyBoon on AVForums.com for the downgrade tip.