For both ELSPA and USK they changed (respectively once and twice) the “stamp” (for lack of a better word?), not the meaning of it, as far as I know.
First I thought the rule of thumb was if the meaning remains unchanged, items must use the newer version, since, say, a „USK ab 18” from 2023 expectedly conveys the exact same meaning as a „USK Nicht geeignet unter 18 Jahren” from 1994, therefore a 1994 game with the latter USK rating should get a “USK - 18” rating as available in the DB.
For what I understand now, this is not the case and theoretically only games displaying the same stamp as the one that is in the DB for the rating entry should receive the according rating (presumably only german games from 2009 for USK).
Yet, there is still the exception kept for USK 6, 12, 16 from 2003 to 2009, which imho seems kinda half-baked between the two choices and is doomed to confuse any editor (honestly it got me quite confused why I got some changes of USK ratings from this period accepted for some entries and rejected for others).