On some older -300's we have the possibility to select manually on the FMC/CDU to either go with 20 or 22k thrust in case of performance problems. but I don't think you can reselect it in flight.
On our NG's the Engine itself is rated for 27k thrust but by putting some chips in the engine, the beast assumes just 20k thrust, hence all perf manuals are all for 20k thrust aswell.
this to increase engine life cycle and most of the time the 73 climbs like a rocket anyway on 2 x 20k.
pressing TOGA just sets the Assumed Temp Thrust over a 20k derated engine