Van de Graffs are a charge separating device. The action of the belt over the rollers causes charge to be separated between the bottom and top rollers. In the “normal” configuration with the nylon roller at the top, the ball stores the +ve charge and the -ve one is connected to earth. But, with the 400mm Van de Graff it is possible to swap the two rollers around. So now the PTFE coated roller is at the top and the Nylon one is at the bottom connected to ground. In this configuration you have -ve charge on the top.
Give this a try, do the sparks look any different ? are the sparks bigger or smaller.
Plus, if you have a second 400mm Van De graff, you can set one for +ve charging and one for -ve charging, stand them close together and get 100mm+ sparks !