Railways can do 2 levels of upgradation, but the same passenger cannot get upgraded across 2 levels. For example, one passenger is pushed up from 3AC (original class) to 2 AC (upgraded class) and another passenger is pushed up from SL (original class) to 3 AC (upgraded class). This passenger WILL NOT BE UPGRADED from SL (original class) to 2 AC (upgraded class), bypassing 3 AC (intermediate class).