If I am someone who roams around the world using my blackberry 8100 by simply installing the local SIM cards where ever I go, how do I update my OS from it's current 4.2.0.67? Which "carrier" do I use to find the appropriate operating system? I've been to the FAQ section under "upgrading the OS" but it's confusing. Do I take an Italian one? a North American AT&T one? The Hong Kong one?...sorry, but this seems like quite the process. What difference does it make what carrier you take if you are not on any one plan? any helpful insight? Thanks

There is no connection or requirement between your carrier or SIM and the BB OS you use.

So, use the latest, always linked in our 81xx section in the Sticky Notes at the top of the page:
http://www.blackberryforums.com/gene...ilanguage.html

You will need your device unlocked to change carriers from the branded one:
How do I unlock my BlackBerry? - BlackBerryFAQ

to updat your os all you have to do is find the new version for your model the cell provider does not matter all you have to make sure is your installing the right one for your model

I understand that..thanks. What if I don't like the upgrade, is it possible to go back to my original 4.2.0.67? Is there a way to back up my old OS?

Yes, there is. Don't delete it from your PC if you loaded it initially from your PC. If not there are older OS downloads available at many of the carrier websites.
How do I downgrade the OS on my BlackBerry? - BlackBerryFAQ