High voltage AC-AC or DC-DC buck conversion
Normal buck converters work with DC.  Since the transformer just feeds a bridge rectifier all you need to do is to do the variable buck conversion after the rectifier.

If efficiency is not a concern why not just leave it in the 15W setting?

What is the purpose of doing this change?  What will it do that it doesn't do now?

This circuit I found looks like it could be adapted to your needs.  This is a linear DC voltage regulator.  It should be incorporated after C25. It will need a low voltage supply as well to power the opamp. As designed it could go up to ~600V. If you change the value of R7 you can alter the voltage range (470K total would do about 300V). R2 could be reduced to a single 470K as well.  M1 should be put on a heatsink appropriate to dissipate 10-15W.



Bartola Valves