Fly_guy100
With regards to economy crew working in business, yes, you may be asked to work there, but you usually get asked in briefing first and even then it's the most senior economy crew who gets shouldered the work. I've only done it once, and even though you don't get rostered the flight as a business class crew member, you can look and see how many Business class crew are short and guess whether or not you're going to get picked. You also get paid the J/C flying pay rate :-D
You are allowed to work one grade above, and one grade below your current position on a flight, ie, economy crew cannot go straight to first class. The only exception is from Business to Senior Flight Steward if the crew member is going for their SFS promotion. Or when it's a high density aircraft.
Hope that explains things.