AirTran ( AAI) downgraded at Bear Stearns to Peer Perform from Outperform due to limited earnings growth. See potential multiple contraction on slower growth in current environment. Removed $16 price target. Alliant Techsystems ( ATK) downgraded at FBR to a Market Perform rating due to deal financing overhang after announcement of acquisition of MDA. Avis Budget ( CAR) upgraded at Morgan Stanley to Equal-weight. Stock is down 65% from its 2007 peak, and cost-cutting efforts should benefit margins in the long term. $12 price target. Centene ( CNC) downgraded at Bear Stearns to Peer Perform rating as they believe the company has closed the valuation gap with the other pure plays in their sector. Delta Air Lines ( DAL) upgraded at Bear Stearns to Outperform from Peer Perform based on valuation. Note shares are trading at 25% discount to sector, with 10% free cash flow yield. Established $20 price target. Delta Air Lines upgraded at Morgan Stanley to Overweight. Stock offers the most upside potential in the group, despite the likely need for downward earnings revisions in 2008. Express Scripts ( ESRX) downgraded at Wachovia to Market Perform. Stock is up 150% since late 2006, and now looks fully valued at 28 times expected earnings. Garmin ( GRMN) downgraded at Deutsche Bank to a Hold rating after checks at CES indicated competitive pressure.