It's not too hard, if you know what you're doing, there is a guide in the help section somewhere, that's what I followed. To be honest I didn't realise that the case came apart so easily. I didn't even use a tool to get it off. Once the connector was off, which just pulls out, the case was easily pulled into two sections by hand, although I would probably use a plastic tool to help get it separated. It might be kinder to the plastic tabs, of which I managed to snap a couple in my haste to get into the thing.
Yes, it would have been better if it was possible to downgrade the firmware over USB, but I've had the drive a while so in for a penny, in for a pound, as they say, and took the plunge.
The hardest part was finding out my laptop doesn't support the older tech for the SATA port and then having to take my wife's laptop apart (it's only slightly older than mine but had the compatibility for SATA IDE that mine didn't) so I could connect directly to the computer. Once that was done it was a case of clicking on the Downgrade button and waiting a few minutes for the software to do its thing.
My plan was for the actual drive itself to live in my laptop, replacing the DVD burner, but it wasn't being recognised, so stuck it back into the case.
Saying that, now it's back in the case it does leave me open to upgrading my laptop to something more powerful!
If the tool doesn't support USB downgrades in the near future and you have any questions please let me know and I'll see if I can answer them.