Adding an action to set the global variable to some other value...
system set global variable to (any number that doesn't have an event attached)
You probably have an event that sets the global variable to trigger this event,
so after the event has triggered, you only have to set it back to the previous value..
Without looking at your capx or all of your events concerning these actions it's hard to make any further recommendations.
The way I usually handle these conditions, it wouldn't be necessary to check for them every tick, because at some point the change of the global variable is triggered and these conditions can be put as a subevent of that triggered event..