To answer the question for those wanting to revert, power the deck down, then hold the … button and power it up. Select the second option (B) and your deck will load back up in the previous version.

Update: So I finally stuck with the 3.5.5 update because after trial and error here is what happens:

Upon reboot: Card is not recognized, requires formatting.

Click and make it go to sleep and awaken again: Card is fine, or is fine with a duplicate entry.

This is a good card, and has been the same one since I got the device. This doesnt give me great confidence, so I am going to back it up when I get home, but I will continue using it and see how it works out.

For those wondering: I will use clonezilla and make an image archive of the whole device rather than simply coping the files.