The relationship between the occurrence of a past event and the second it occurred can not be changed, barring relativity. As such, storing the absolute second offset unambiguously persists the actual time the event occurred regardless of any legal changes.

It will just convert to a different, now agreed upon, civil time according to the calendar chosen. It is no different from citing the current second according to the Gregorian calendar versus the Mayan calendar. They both refer to the same “time”, but use different civil dates. The underlying thing that is convertible to different calendar formats is the real thing and corresponds to the absolute time of the past event.

edit: And no, it is almost guaranteed that most past time events are timestamps of the “current” time which has the same qualities as “past” time. Only future civil times should not use absolute second offset times.