how economy would change if AI replace whole workforce?

I believe in that case, the best case for humanity would be modified socialism. 

In a capitalist world, people who do not own any corporation that creates physical things would have absolutely nothing. The only way to gain money would be in exchange for resources, and people who can produce the most resources would be the wealthiest, and the wealth would only rise exponentially, while average person would stay in place and try to survive. 

Definitely here there would be rebellions and attempts to force the corporation to find some way to tax them and, when giving free money, for each person. or they will create massive gangs of people in order to steal resources from wealthy people. The capitalist world would probably turn into modified socialism instead of "socialism is a political and economic system where the people collectively own or control the means of production." It would be "socialism is a political and economic system where the people collectively control every single means of production in the entire globe," extended democracy. 

yes buddy but what if it happened? 
that is pretty interesting thing to debate