The Securities and Exchange Commission has shifted its probe of KB Home ( KBH) from an informal inquiry to a formal investigation, the homebuilder disclosed Friday. KB Home disclosed August that the SEC had launched an informal probe into its stock-option granting practices. Last year, the company uncovered a massive stock-option backdating scheme that resulted in the resignation of longtime CEO Bruce Karatz in November. Los Angeles-based KB Home said in an SEC filing that it has cooperated with the SEC on the matter and it intends to continue to do so. Shares of KB Home were down 20 cents to $51.92 in premarket trading.