Yes, with low slung engines, the aircraft does get a pitch up moment from increasing thrust, however in FLCH (with the autopilot engaged or manually flying following flight director guidance), the aircraft will pitch to maintain the selected speed.
If the thrust is at idle you will descend. If the thrust is higher you will descend more slowly. If thrust is increased further, the aircraft will level off and with still more thrust will begin to climb. It ain't rocket surgery or brain science!