If its indeed a Y fare full price coach ticket (not just a coach ticket in some other class of booking), you can upgrade for 15000 miles one way from the US to AMS, 30000 roundtrip, provided there are seats available in the upgrade bucket. Not sure about specifics on AMS-Africa portion. Call the airline to check. If not, op-ups are VERY rare for Silvers, i wouldn't hold my breath.