For upgrades the general rule is you can only upgrade a flight marketed (flight number) and operated (metal) by the airline of your frequent flyer programme (no codeshares) with your airlines upgrade instrument (e.g. SWU, miles) or complementary with status (if applicable). All subject to the t&c's of your FFP and your paid ticket. Low cost tickets are often not eligible for upgrades.
However as is the way with FFP's there are some upgrade exceptions
- on some Star Alliance airlines with higher priced Y & B fare

This may be of interest http://www.flyertalk.com/forum/trave...demptions.html
http://www.economicalexcursionists.com/travelTools/ Does not have all airlinbes ffp and may not be up to date