I'm not someone to argue on the forums, but that just seems like a bad idea. Having a bunch of level 20-30 managers have their supply frozen just seems like something that would make those people confused and mad. Surely there has to be another alternative that can be thought of.
Not doing so could confuse and anger those who are level 30+, those who have more game time and expect the few things that the highest level is now needed for, to not be left available to those who shouldn't be due to an oversight. Asking people to wait until level 30 (it's been written for about a year in the xp progress page that this would be the case, so they can't say they didn't know) doesn't seem unreasonable to me. I add that the research % given by the additional hubs could be too much of an advantage for levels 20-29, given the research formula which already gives them a higher % than levels 30+.
I also support whatever decision is made, but I would expect the original intention to be respected. By freezing they would not lose any of their progress and would not have to spend the money again, it would be reactivated for free once they reached level 30 and they would continue the tech tree from where they left off.