While upgrading at check-in for a discount seemed to be the trend a year ago on these boards, I haven't seen too much of that posted recently. In fact, I've seen more people reporting that concierge levels were selling out.
We did try to upgrade in May 2004 for one night at the GF. They had availability, but we were quoted the difference between rack rates. No offer of savings there.
It doesn't hurt to try though.
Tammi