Have done a number of concierge and even more regular
DCL cruises and agree that you can definitely request and should be able to get a switch to earlier seating. The question I have is really, why there is such a push to get the earlier seating. (OP you do not have to explain, it is a more general question for any one reading this thread.)
We typically have done the later seating, and usually move to later if not assigned, after having experienced both. Sure it might be different from our normal routine but we are on vacation. Our son did well with late dining over the years. One cruise with the cousins where we did early, things did not go all that well. Kids had afternoon naps interrupted or missed with the port and dinner schedule. We find it easy to have a more relaxed afternoon pace with late seating. If anyone is hungry there are plenty of snacks around the pool deck and particularly in the lounge when sailing concierge.
Not saying don't try and some people it may not be ideal still to have late dining. Just sharing be open, and if you don't get it you may be pleasantly surprised.