This question already has an answer here:
Why B-52 was not re-engined as a more efficient (also for maintenance) 4-engine but is still an 8-engine aircraft?
In history, almost all others models were re-engined with new engines also quite different from original, some examples could be C5 Galaxy or also B737 that in latest geneations requires also to extend undercarriage or make bottom "flat" area of engines, to accomodate new bigger ones.
So why B-52 remains the original projects despite its long history? Did development cost be affordable in this very long period?