Hello. I am a new flyer to American Airlines and I am a bit confused on how upgrades work? I see they sell 500 mile upgrades that can be used to upgrade your seat. I believe I understand how that process works. But my question is, if you are an American Silver or Gold, are there no complimentary upgrades? Do you always have to purchse the 500 mile upgrade package to be upgraded? I see that Platinum receive complimentary upgrades if available, so no worries there.