Their powers changed because the marigold wasn’t in them for all those years. Their ‘potential powers’ within them drifted, and they were only locked in to their new powers when they drank the marigold. In other words, their powers can only change while they don’t have them - when they have powers, they stay the same (but can be improved a lot through training as we’ve seen).

I don’t think being mixed with another drink has anything to do with it