I don't fly a boeing, but E170/E190's.
The posters question seems pretty reasonable. We have TO-1, TO-2 and TO-3, all of which we can flex.
Most of the guys in work are surprised to learn that if we use TO-3 with a flex, it does not matter how hard or how far we push the thrust levers, we are not going to get any more thrust than TO-3 no flex, with 2 exceptions.
1. Windshear will give WSHR thrust.
2. Engine failure will give us Reserve power, but only TO-3 reserve.
If we have completley messed up our performance, and the end of the runway, or an obstacle is rapidly approaching and we wan't to get away from TO-3 flex to TO-1, then some very quick FMS programming would be required.