I am going to be flying YYZ-LHR this summer and would like to upgrade one way. I was wondering if someone could explain to me the difference between SWU and SSWU. What fare do I have to be booked in in order to upgrade, and are there any other restrictions? Also, how can I get my hands on a SWU/SSWU? I checked ebay but no one's selling right now Thanks for the help!!!