IBM Corp may have persuaded Wall Street that it is truly on the mend, but Standard & Poor’s Corp and Moody’s Investors Service Inc still have to be convinced. While the planned repurchase of preference shares should cut IBM’s costs, it will do nothing to raise IBM’s credit ratings, according to both credit-rating agencies. Analysts at both companies told the Wall Street Journal that the move made sense given IBM’s cash holding, but added that they are looking for more concrete signs that IBM’s turnaround is permanent before they raise ratings.