When you press FL CHG or VNAV the autothrottle enters a "combined mode" - in most cases it will set climb thrust (N1) to fly the airplane to the next desired altitude. Try this - put the MCP altitude to below the aircraft and then click FL CHG. You will see that the autothrottle enters the combined mode of "retard - idle" and tries to descend.
So yes, on the older software FMCs the throttles would advance to N1 limit thrust, and if the pilot is a total fluke, it would "take the aircraft to the air" (unless buildings or trees are in the way)
The autopilot has no idea wether you are on the ground or not, you can even see it trying to fly a turn if you engage HDG SEL when you engage CMD on the ground...
Jan