Fro SWUs, W(V, H, M, U, B, Y, etc.) & higher fares are required for international travel. Do not book an S,T,K,or L fare. If using a SWU, try and keep the metal the same if you are waitilisting for your flights as you can not waitlist both a CO and UA segment at the same time. If you can confirm the upgrade at booking, then one segment UA & one segment CO is fine. My advice is to keep the metal the same when flying in one direction.