Guys, some of you are mixing flex/assumed temperature method of thrust reduction with derate. They are different ways to reduce your thrust.
Assumed temp/flex means that you reduce the thrust by assuming that you have a higher temperature. An advancement of the thrust levers after T/O is OK as your T/O speeds are taken care of.
Derate means that you operate the engine at a fixed percentage of thrust reduction. It's like bolting on a smaller engine. You can also use flex/assumed temperature on a derate.
Problems occur if you want extra thrust after T/O as your speeds have been calculated on the derate. You may not have sufficient rudder authority as the derated speeds are lower than using flex/assumed by itself.
Oz