Alcoa (AA) could be a solid choice for investors given its recent upgrade to a Zacks Rank #2 (Buy). An upward trend in earnings estimates -- one of the most powerful forces impacting stock prices -- has triggered this rating change.The Zacks rating relies solely on a company's changing earnings picture. It tracks EPS estimates for the current and following years from the sell-side analysts covering the stock through a consensus measure -- the Zacks Consensus Estimate.Individual investors often find it hard ...