It might still be possible to change, by changing the macros that unwrap integers to support tagged integers, then waiting a decade or two before actually start

It might still be possible to change, by changing the macros that unwrap integers to support tagged integers, then waiting a decade or two before actually starting to use tagged integers. Things that hadn't been recompiled in a decade or two, but were for some reason running the latest interpreter would break. Presumably the number of breakages would be negligible.