You can downgrade the firmware that way, but any baseband updates that are installed (on an iPhone) are there forever - you can't downgrade to a previous baseband that is jailbreakable/unlockable.
This is only relevant to iPhones, since it just means that you can't use earlier unlock tricks to unlock more recent basebands. You can still jailbreak, though.
_________________________ If it's brokenless, don't suffix it...
Shouldn't matter which iTunes platform you're working with. You can download older .ipsw's off the internet - just make sure you get a reputable one. There are quite a lot hosted out there, sorry I can't point you to one right now. I remember finding mine by searching for links in jailbreak forums - people will tell you where the gooduns are. Then you just have to drop it in the same folder as your existing .ipsws, and do whatever the winxx equivalent is of option-click on the Restore button in iTunes, which then offers you the chance to choose your .ipsw file.
_________________________ If it's brokenless, don't suffix it...