Is/why isn't the Antonov 124 to 225 wing upgrade method common?
With straight wings the root extension method is very common. Airplanes that come to mind are

Me-109 H (high altitude version with increased span)
Beech Twin Bonanza and Model 65 Queen Air (used the Bonanza wing from the engines out)
SB-10 (uses SB-9 wings stuck to a new carbon-epoxy center wing).

This is impractical with swept wings because of the strong twist and airfoil changes which are needed at the wing root to keep the isobars straight on the swept wings. The only way to increase an existing wing's size was used for the A340-500/600 by inserting a triangular section at mid-chord into the A340-200/300 wing which also allowed to increase its wingspan.


Source: Evolvability and design reuse in civil jet transport aircraft