The answer to the original question is at lift off.
At weight off wheels you'll notice the take off ref page swap to the climb page. At this point there is no further reference to derate (ie your 20k derated engines are 22k again).The thrust levers are in THR HLD (ARM at 800'), then the next setting the A/T sets is N1 at 1500' which will be based on your chosen climb(climb,-1,-2) setting. This may even mean increasing thrust if a high assumed temp was used for TOFF. It is this reduced climb which has the upper limit of 15,000'.
Hope this helps
HJ