Assuming EECs are in normal mode and autothrottle is armed for departure, upon pressing a TOGA switch the autothrottle will set the FMC-selected reference thrust (ie the target takeoff EPR for RRs and PWs). This will be done by 65 kts, at which point the autothrottle servos will disconnect ("HOLD" mode) and provided neither pilot touches the throttles, the EECs will ensure that the EPR that was set remains constant. From our FCOM 1:
When the engine is stabilized, EEC keeps thrust constant independent of outside air temperature and pressure.
At 400' the autothrottle will re-engage ("THR REF" mode) and when the reference EPR changes (normally at thrust reduction altitude), the thrust levers will be moved accordingly. So in answer to your question, from setting takeoff thrust until at least 400' AAL when the A/T re-engages, the EPRs shouldn't change.
MD
PS Speaking of higher ZFWs on the freighters, on our 747-400ERFs normal MAXZFW is 277,144 kgs and can even go as high as 288,031 kgs albeit with a lowering of the MTOW...
