Is it possible to do half power or switchable wattage mods to any amp? What about to an Orange Rocker 30?
"There's Jimmy Page, the greatest thief of American black music who ever walked the earth."
-Homer Simpson
The short answer is yes you can... the problem is that if you think a half power switch will make that much of a difference in volume you are incorrect. If you are going to mod an amp the better solution would be to either install a master volume control, in particular a PPIMV, or to buy an attenuator.

A PPIMV allows you to crank the preamp and adjust the signal going to the power amp.

An attenuator allows you to crank the amp, and then adjust the signal going to the speaker.

Any volume reducing method will have some effect upon the tone due to how the human ear perceives sounds.