Yes, but Boeing built 744 of the things, and only 78 (or fewer) are still in USAF service (I believe NASA has one …). So the ones being upgraded are from a late production batch (the B-52H) and presumably by that point Boeing had standardized things properly.

Comet production ran to 114 aircraft in four major versions, of significantly different size and engine configuration (they started with a 36 seater, and ended up with 119 passengers in a charter-variant of the Comet 4C). No individual model of Comet got anywhere close to a triple-digit production run.

Oh, and 26 hull loss accidents!