Wiring and avionics and such, yes, but the structure is still original. This matters because of metal fatigue, which means that the structure is getting worse by the year.
Semi-modern inspection techniques (eddy current inspection) and X-ray inspection can detect hidden defects and cracks and aluminum airframes are fairly readily repairable.
I wouldn't expect that metal fatigue will prove to be an impediment to the B-52's ability to continue to deliver ordinance.
It would seem to me that if they didn't have fancy CAE (Computer Aided Engineering) and CAD packages to precisely calculate tollerances they would over-estimate and overbuild to be on the safe side. Perhaps as a positive side-effect is that those frame would last longer because of that.
