I didn't think you could upgrade international trips except with miles or ExcePlat certificates. Domestic upgrades which can be purchased or earned are not combinable for travel outside NAmerica. Only way would be to build up enough earned threshold upgrades to convert them to points, then cash the points in for the appropriate upgrade award, number o f miles required being based on the fare of the original ticket.
Otherwise, you are at the luck of the draw: if you flight is oversold in Y, you may be upgraded based on your status, but there are likely to be Plats and ExecPlats ahead of you. And getting two seats upgraded on a deeply discounted ticket at Gold status may also be tricky, though not impossible.