As a new Mk is being introduced, a new Aircraft Safety Case is required; as it would be even if the role of the aircraft changed. (And, from the various posts here, that does seem to have changed in recent years for the Mk2, so I assume its Safety Case is wholly up to date).
It may be retained, and may carry the caveat "it was built to standards of the day", but that does not infer automatic read-across. Under the circumstances I think I'd be ensuring it was brought up to latest standards. Perhaps that is what has caused the 7 year, and counting, delay?