Wouldn't that change be already priced in?
Um, no.
Are you citing "efficient market hypothesis", which is the most wrong scam of an economic theory to ever exist?
"The efficient market hypothesis (EMH) is an investment theory that states it is impossible to "beat the market" because stock market efficiency causes existing share prices to always incorporate and reflect all relevant information."
According to that theory, it's not possible for millionaires to exist who made money in the stock market. It pretends there is no asymmetric information distribution or capability to analyze such data and that all humans are 100% optimized branch predictors LOL.
I always thought EMH was the emergent behaviour of the market - e.g. the current price.
Participants all buy/sell, some win some lose, the distribution of winners losers is some statistical curve, normal, Poisson, whatever.
I don't think winners and losers is mutually exclusive with EMH.