I know there is a topic about this, but I didn't see it when I looked for it.
My idea about this if it does get implemented is whenever you choose to change a passive you get your level reduced by 20 and choose a stat to reduce by 20 levels as well, then you have to retrain those twenty levels while the exp requirement stays what it was and goes up as normal so it's harder to get that 20 levels back. There is a time limit of once per day you can change a passive and you have to be level 120 to do it again even if you wait the full day.
Edit 1: Using the change also resets the exp you have to 0 so people who stacked can't instantly get cap again, costs 25 million Ryo to do as well.