Aetna ( AET) downgraded at Merrill Lynch from Buy to Neutral. Valuation call, as the stock is up 20% over the past six months. Baker Hughes ( BHI) downgraded at Friedman, Billings Ramsey to Market Perform. $96 price target. Valuation looks rich, and the company is growing slower than its peers. Cardinal Health ( CAH) upgraded at Credit Suisse to Outperform from Neutral based on valuation. Sees favorable risk/reward for the stock. Raised target price to $77 from $74. Hartford Financial ( HIG) downgraded at Keefe, Bruyette & Woods from Outperform to Market Perform. Valuation call, given the $109 price target. Price competition is already heating up. Harvest Natural Resources ( HNR) upgraded to Buy rating from Hold at Jefferies. Price target rockets to $17 from $11 and FY 2008 EPS estimates hold at $1.06. ManTech International ( MANT) upgraded at Cowen to Outperform rating from Marketperform. FY 2008 EPS estimates raised to $2.15 from $2.14. Northeast Utilities ( NU) upgraded at Merrill Lynch to Buy. Stock has pulled back 13% from its April highs. Fundamental growth remains strong, and transmission assets are gaining value. Office Depot ( ODP) downgraded at JP Morgan from Overweight to Neutral. Company delayed its third quarter earnings, because of an internal vendor review from its audit community. Office Depot downgraded at Credit Suisse to Underperform from Neutral after company announced the delayed release of Q3 earnings. Lowered target price to $17 from $24.