I'm going to switch obese people's drink at mcdonald's
What is your reason for wanting to switch the drinks of obese people?
I like doing irritating things just to be evil.
You need to just leave their drinks alone. There is no reason to mess with other people's drinks like that, and I'm sure you'll cause them to become enraged.
You obviously don't know some people. They should smile and be happy. They should count their lucky stars they are being served soda. If they cannot be happy with that they need to seek help.
Messing with someone's drink is simply bad news. They may think that you are doing something to it and trying to hurt them. You are better off leaving other peoples things alone.