Shares of Marvel Entertainment were up Wednesday after Morgan Stanley analyst Kurt Feuerman upgraded the stock to a “buy” from a “hold.”Marvel gained $ 1.875 per share to close at $ 29.25, a 15% jump. Volume was heavy with 275,800 shares changing hands. Feuerman also increased his 1994 earnings estimate to $ 1.45 a share from $ 1 .40 but left the 1993 estimate of $ 1.08 unchanged. Marvel posted profits of 66 cents per share in 1992. Sources say Feuerman told clients that Marvel’s business momentum continues to be strong. He said that Marvel management is working aggressively to enhance the value of its 3,000 characters through licensing and merchandising deals and that the company is building off its comic book strength to create a youth entertainment company.