BAH stays with you as long as you stay with the military. For BAH, you maintain "uninterrupted eligibility" with protection from any potential reductions. This means you are authorized the January 1st BAH rate for your grade and location, or the BAH rate being paid on December 31st of the previous year, whichever is larger.
But there are three things that may lead to a change in your status which then "interrupts your eligibility" for BAH allowance (and then, only if the published allowance for your grade and location is less than what you are getting now).
1. PCS and BAH: First, and most common, a BAH decrease occurs when you PCS to a location at which the housing cost is less than at your current duty station. At the new duty station you get a lower housing allowance, but you should be no worse off, because the allowance is driven by the housing costs there.
2. Demotion and BAH: Second, if you are demoted, your BAH housing allowance reverts to the then-current published table of allowances for your lower grade. Promotions do not lower your BAH allowance.
3. BAH and Your Dependents: Third, if your dependency status changes (from with-dependent to without-dependent, or vice versa), your BAH allowance will be determined by your new dependency status at the current published allowances for your grade.
If you are promoted, your BAH rate is the then-current published BAH for your new (higher) grade, with the following exception. If you get promoted, and are in a location where the current published BAH for your new grade is lower than the BAH amount you were receiving before, you continue to receive the higher BAH amount.