Investors have ample reason to be sour on Brazil. Falling commodity demand from China combined with overly indebted consumers at home has helped slice the country's annual economic growth rate from 7.5% to 2.5% over the past three years. And while the S&P 500 has racked up a 22% gain so far this year, Brazil's benchmark Bovespa stock index has fallen by 12%. But some market observers say the selloff has created bargains. And a recently announced change to the way in which the Bovespa will be calculated could give an additional boost to high-quality stocks -- creating a rare buying opportunity.