If you push the TO/GA switches after take-off LNAV/VNAV will disarm, TO/GA will engage, ALL derates will be removed and the autothrottle will engage in THR REF.
If you don't push the TO/GA switches and increase thrust by manually advancing the thrust levers, the autothrottle will activate when VNAV engages at 400ft above runway elevation, and set the thrust to the selected take-off thrust setting. That will probably result in a thrust reduction - not what you really need at that point!