Most o/u stocks can be bent to change the cast - providing part of the
cast does not include rotating the butt of the stock to put the bottom
"toe" of the stock closer to the armpit. Even then, depending on the
grain in the stock, it may be able to be bent successfully.
You would find it very difficult to impossible to use a good shooting form
(gun mount, stance, head/neck/upper body posture shooting a gun with
a stock that has the wrong cast. Depending on the cast of the stock, a
right-handed stock could really corrupt your gun mount and make it
difficult to align your eye with the rib and keep it aligned during swings
As was written above, many left-handed shooters shoot guns that have
no (neutral) cast. It just requires a slightly more rotated stance toward
the side of the gun mount (counterclockwise for a left-handed shooter.