The attack you are suggesting is virtually impossible. What motive would people have for accepting a client that was rigged in such a way, particularly if the utility of bitcoin for them was harmed by such a choice? All that would happen if the developers did that is that their client version would quickly cease to be the official one.
And nothing stops anyone from writing their own client. So long as it follows the same network protocol, nobody need even ever know.
Surely there is also the issue that some users are going to be using the older client, at least initially. So if they release a new client with this rather major change to the system in it, the vast majority of users would initially be rejecting those blocks that didn't follow the old rules. At this point it would become clear that there is something bad with the new client, that it was issuing bad blocks.
So one would have to ensure that it was hidden in the new client, but didn't start to create bad blocks until some point in the future when most if not all users would have upgraded. And being open source, its unlikely that such code could escape notice for long.