Yes, it is. The current H generation has very little to do with the first build airframes so. Including certification.
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.
That's a little disingenuous, regarding the F-15 -- there is the A/B series, with A being single-seat and B being the two-seat trainer, and the C/D series, with the same distinction between the two models. Both the F-15A and the F-15C are air superiority fighters, with an exclusively air-to-air role. The F-15E is a multirole fighter-bomber, not an air superiority fighter. The F-15X is an F-15E repurposed as a beyond visual range (BVR) missile truck, with data links to AEWC planes that can provide targeting solutions for those missiles. All of the letters between E and X have not been used.
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.