27 C.F.R. § 19.167 – Increase of bond coverage.

(a) When required. If the penal sum of a bond is less than the maximum amount specified by § 19.166, and liabilities increase to the point where they exceed the bond coverage, the proprietor must increase the amount of the bond to cover the increased liability. The proprietor must increase the bond coverage either by replacing the existing bond with a new, larger bond that covers the entire liability, or by supplementing the existing bond with a separate strengthening bond in accordance with paragraph (b) of this section.
(b) Strengthening bonds. A strengthening bond is a second bond with the same surety as on the original bond which covers the increased liability. A strengthening bond must show both its execution date and its effective date. TTB will not accept a strengthening bond if it contains any term or condition that is a release, or could be interpreted as a release, from liability under any former bond, or that limits the liability of any bond to less than its full penal sum.
(26 U.S.C. 5173)