Effective Sept. 1, Fannie Mae is prohibiting lenders who sell it loans from changing appraisers’ numbers. In guidance issued June 30, Fannie Mae said lenders must contact appraisers to resolve any disagreements about the valuation. If that’s not possible, they should order a second appraisal — not just chop the value supporting the real estate contract. Appraisers applauded the new rule. “This is huge,” said Gary Crabtree, president of Affiliated Appraisers of Bakersfield and a member of the national government relations committee of the Appraisal Institute, an industry group.