ExlService Holdings (EXLS) could be a solid addition to your portfolio given its recent upgrade to a Zacks Rank #2 (Buy). This rating change essentially reflects an upward trend in earnings estimates -- one of the most powerful forces impacting stock prices.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.Since a changi ...