At 145 lbs, the 125 liter board will seem "challenging" to uphaul right at first, compared to your longboard, but you can/will quickly adapt to the slightly different skill set it takes to uphaul.
It has plenty of reserve volume for a sailor of your weight.
At your weight, with a 7.5-8.5 m2 rig, you may start to plane in around 12 knots of wind (maybe
slightly less with an 8.5 m2 free race rig with lots of draft and as you develop goode pumping
skills on the 125 liter shortboard.
In sub planing condtions for the 125 liter shortboard, you will be much better off on your longboard.
You will need to "rail" the short board alot just to stay upwind and the railing will prevent you from planing to some degree.
You can certainly sail the 125 liter board in < planing windspeeds, but it won't be very much fun when compared to planing @ 12 knots on the same board.