Not just a total rewrite of the code, each and every
scenario would need to be reengineered, including those buried inside the campaign games. Also
every save game on everyone's own hard drive would need an updater run manually by them at thier end. Any PBEM games in progress would be toasted (normal for any update).
Needless to say umpteen end users then would
completely ignore any message on the installer (even if in red text and a pop-up alert was displayed to that effect) and then they would write in with "my save game exploded!" type messages...
So,
not going to happen. A change to the basic game data structures
so fundemental would really require a brand new game from the ground up, with all new scenarios and campaigns being written just for it as a rewrite is likely to miss bits leaving "gotchas". And it would sold separately of course!.
Likelyhood of that happening is
vanishingly small.