A fatigue-life part in the BUFF is some sort of "Wing clip," which was frequently touted as a success story for AMARC (the boneyard at D-M).

I believe the "Clip" is a major structural component that ties together the wing boxes in the fuselage.

On tours of the boneyard, the AMARC guide would tell how early model BUFFS would have the part salvaged, and installed on the current fleet, as it was otherwise unavailable, and without it, the aircraft were unflyable.

I'm not sure if the current fleet of B-52Hs are included, but earlier models I know were beefed-up with reinforcing straps along the bottom-side of the fuselage (almost the full length of the fuselage), due to higher stresses from flying low-level penetrations (the airplane was designed to be a high-altitude bomber).

A similar beef-up was done to T-38s about 25 years ago, closer to the top of the fuselage, for fatigue resistance.

Other than that, I know of no major structural work on the B-52H. KC-135s had lower wing-skins replaced in the 1980s; I think they are about to do it again. Don't think that has ever happened with the BUFF.