Are you interested in the app displaying the right voltage numbers? For anything else (aka nice graphs from logged data), you can simply do voltage*20/16 (so voltage*1.25) to get from the 16s 67.2V to the 20s 84V numbers; and *16/20 (*0.8) for the reverse (which is what the Gotway firmware does, faking 67.2V numbers for a 84V wheel so they didn't have to modify their app to support 84V wheels).