Not being very knowledgeable on auto boxes, did a bit of reading and there are a few things that can be done...
The auto box *should* be a Jetco 4N71 variant, the L4N71B.
First and easiest thing is to modify the throttle vacuum valve's rod. According to
this article, a mod like this can alter shift patterns.
Second thing to do is install a "shift kit" into the valve body (gearbox's hydraulic "ECU"). This gives faster but slightly harder shifts and supposedly allow gearbox to cope with higher powered engines due to increased oil pressure.
http://www.viczcar.com/forum/index.php?topic=6869.0
Third thing is to convert the valve body to "full manual", allowing you to shift as a clutchless manual. Americans like to do it for racing!
This would change the gear selector from PRND21 to PRN123. There's supposedly a way to make it semi-auto to but have not found much on either with regard to the S12 box.