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.