Assuming this is a typical T class fare...change fee + any fare difference + any new taxes/surcharges if they've gone up. You will lose the upgrade unless the flight you move to has award space too which means you can keep it. If it doesn't have award space, you should get the miles back for that leg.

This is all pre-departure. After departure, if you change the return leg you'll lose the upgrade full stop (and no miles back either).