I just did this to one of mine to see the effect of what happens. The operating rod can't really go anywhere if you move the lever too far. It will be stopped by the metal endcap that covers the back end of the upper handguard. This metal cap has a small hole for the gas rod to go through but will not allow the operating rod to move out with the gas tube in place.

I would return the lever to the normal position for removing the gas tube and just use a little upward force to remove the gas tube and upper handguard assembly. Careful, the op rod may launch. If so, just push back into place and all is good.