I have a moen one handle shower controller... installed backwards, evidently (by the previous owners who were plumbing contractors!)... so when the indicator is pointed to hot you get cold water, and when it's pointed to cold you get hot water.

What's the easiest way to resolve this? I'm wondering if I can install the handle upside down... or install the trim ring with the blue and red color indicators upside down? Would be nice to resolve without digging into the pipes...

Remove the handle. Grasp the stem of the cartridge with a pair of pliers and turn 180 degrees. Reinstall handle.