Earlier quoted context omitted.
It's honestly a fantastic case for incremental improvements over time. There's practically no single part on a modern B-52 that's original to the 1960s since everything has been overhauled and modernized decade by decade. Every attempt to replace it has been (or will be) retired before its EOL.
Platonic question: is it still a B-52 then? cf. the Ship of Theseus
EDIT: Those upgrade programs, e.g. the F-15 is currently at "X", are complete development programs and involve a certain amount of certification and re-certification. That in the case of the B-52 those upgrades have been applied to existing, tremendously old, airframes, doesn't change that. It does make it quite impressive so, the B-52 turned out to be a very flexible and upgradeable design.