I have a traxxas slash 4X4 BL-2, Can i change my ESC to a 3s and run 3s batteries without hurting the motor?

The BL-2s is not recommended to run on a 3s battery.

Doesn't the BL-2S motor also have that strange proprietary plug? I suppose you could probably just cut it off and solder on some bullets, but just another thing to have to fiddle with.

You could, but I'm not sure if the leads are marked to indicate which point they are.

I don't think it matters for sensorless motors anyway. You would just have to swap the wires to make sure the motor is spinning in the correct direction.

This is true, but it would be nice so you don't need to desolder and resolder. Then again, if you're using bullet plugs, soldering wouldn't be an issue.