I really don't think they would go after LP's making a living considering their pretty chill stance to YT videos compared to other companies. I don't think they suddenly would start making ass moves like that.No, the ads don't change, but the ad money goes to Nintendo, NOT to the people who uploaded the video. Like Guild stated, lots of people that do LPs (Pewdie, etc.) literally make their living (that is, the money they use to buy the food they eat and pay their rent) from the ads on Youtube.
But they won't be getting it from videos Nintendo does a claim on. The videos stay up, but Youtube sends the money to Nintendo, not the Youtube account's holder.
Just speculation, but I don't think they would want to bring such negativity upon themselves from people who promotes their games for free.