It's not a question of switching to Brazil. China was already buying from Brazil. But Brazil doesn't have the capacity to make up for a cut-off of American supplies.
Further, soybeans are seasonal. The reason that China buys from both the U.S. and Brazil is because they have harvests at opposite times of the year. Even if Brazil managed to magically ramp up to fill 100% of China's need in each Brazilian growing season, what is it going to do the other six months of the year?
This literally already happened as another poster said...
https://www.bloomberg.com/news/articles/2019-08-05/china-ask...
Trump wants a trade agreement with the EU for soy and beef in exchange for not imposing tariffs on cars. It probably won't happen, as the EU is too slow to act and most likely won't accept hormone treated beef.