Derate Levels
Just to muddy the waters slightly, it depends on how the derate is done.
I'll illustrate this with the A320, although the same applies for the 737. Mechanically the same engine can be fitted to any airframe from the 319 to the 321, and the FADEC can be "pin-programmed" via the data entry plug to set the installed thrust level. Pin program for a lower thrust rating, and there is no way the crew can override this.
However, if you enter a derate (or Flex Temp) via the MCDU Perf page, the maximum thrust available is still TOGA thrust.