Attempting to change a Command Block's command will result in it not saving properly. Changing the command block's state (e.g. setting it to Impulse/Chain/Repeating, Conditional/Unconditional, Always Active/Needs Redstone) will work; the command itself will not appear to.

The command will run, however, if set to Repeating Always Active; it will immediately stop once you enter the command block, and Always Active will change to Needs Redstone.