As I can’t find this nicely spelled out anywhere, here are the economy fare class codes that you need to book to ensure you can use your system wide upgrades on any flight: Y, B, M, E, U, H, Q, V, W. And for Business: J, C, D. This is especially true for those of us flying in and out of Australia…
1 http://pss.united.com/web/en-US/content/mileageplus/awards/upgrade/default.aspx
2 http://www.united.com/web/en-US/content/booking/flight/fareClass.aspx