Agree with above. This is not standard practice or policy. Perhaps the payment to SDC was factored into the decision, or your status was, but U is supposed to be available for the upgrade to carry-over. I think in standard practice this even "loses" the GGU, and another cert (or same one reissued) would be required to confirm the space on the new flight, if I'm recalling the one or two times I've done this a few years back correctly.