We had this kind of issue when AAs were first introduced. Characters at cap had the devils own job getting on par with those levelling with the AAs so to speak. While these achievements won't have any real impact on play it would be disingenuous to suggest that it would be fine with everyone if you had to go back and do things a second time just to compare with newly created characters.
I don't mind flirting with a couple of hundred people again but killing Holly Windstalker again just "because" seems sort of pointless......been there, seen that, done it, so to speak.