Interesting observations this morning…
It looks like if you can get a basic fare and need a preferred seat at booking you can upgrade with cash for approximately the same amount as upgrading the fare to choice (which only includes standard seats at booking). Also, if booking with points, upgrading from basic fare to choice seems to give pretty good value for the points (between 1.8 to 2.0 cents per point), but if you were trying to upgrade to choice extra the points values drop below 1 cent per point.
My preferred airport (BNA) however doesn’t even show any basic fares available for all the assigned seat flights released so far. Maybe everybody in Nashville bought them up super early this morning??