I don't remember the firmware version I had for sure, but it was a recent one: my ereader did connect to the internet and did an upgrade the day before I did the downgrade.
Downgrade was done with no trouble. But it took a long time, and at one point I though it was not gonna reboot. It finally booted and I kept following what was indicated on
the wiki. It was few days ago. Unit coming from EU too.
I've been able to upgrade to the latest firmware (5.9.4) later on with no problem.