Originally Posted by Jim C
...Will allow increase / decrease of distortion without changing output volume...
You're typically increasing GAIN when increasing "distortion". Gain adds volume, hence the increase in output volume.
I'm not immediately aware of any pedal that automatically attenuates output volume as you change the gain parameter.
Having multiple pedals set to various gains or a pedal that can store presets, like the Source Audio OFD and other preset capable pedals, would fit the bill.
Alternatively, if you only have two settings you'd like to use, just put a small colored mark on the pedal next to the respective settings for the gain and volume knobs so you know exactly where to turn them for each setting. Costs you nothing other than some effort.