At this point, if someone released a new client with better features and that was backwards compatible with the old clients, I think people just might adopt it in favor of the old client ... even if it also accepted more rapid block creation or whatever.  Especially if this hypothetical client were to be released by Satoshi and put on the main page.  Once we get a fuller ecosystem with a whole bunch of clients (think how many clients implement Bittorrent) that will be hard - if not impossible - to do.

The official rules of bitcoin aren't written in stone, they're written into the clients.  As long as it is possible that 50%+ of the network could switch to a new client, it is possible that bitcoin could radically change.