An export warehouse proprietor shall file a new bond to supersede his current bond, immediately when (a) the corporate surety on the current bond becomes insolvent, (b) the appropriate TTB officer approves a request from the surety on the current bond to terminate his liability under the bond, (c) payment of any liability under a bond is made by the surety thereon, or (d) the appropriate TTB officer considers such a superseding bond necessary for the protection of the revenue. (72 Stat. 1421; 26 U.S.C. 5711)