Hi Drew
I have done the conversion, by swapping barrel,piston, head and carb.
All the bottom end is the same , apart from the final drive sprocket which goes up a tooth on the 150.
The heads are different, combustion chamber is smaller on a 125.
The inlet stub and carb are smaller on a 125, they may work on a 150, but I think it would reduce the power.
The result I was very pleased with, the engine was much smoother when running, ( possibly the engine balances better as a 150), and although performance was not much quicker the whole feel of the motorcycle was that it was not being “thrashed” quite as much. Definitely worth doing.
Hope this helps
Peter