Originally Posted by
gatelouse
One corner case that we haven't studied is where coach is full and only business seats are available. (For Acela, we mean that business is full and only first class is available.) It is unknown whether upgrades will be permitted in this situation, though I don't see why not.
About two years ago I was trying to get on an earlier Acela from Newark to DC, and it was sold out in business. But they let me pay for a business class ticket at the government fare and use my coupon to upgrade to first, which still had space. YMMV