BAH (Basic Allowance for Housing) is an entitlement designed to offset out-of-pocket housing expenses for military families living off base. After PCSing, there may be a difference in BAH between the old duty station and the new duty station. This is because lease rates can vary significantly from one location to another.
Be sure to speak with the housing office at your new duty station before you PCS to get a feel for how much your BAH is likely to change. Additional information can be obtained through the Defense Travel Management Office.