Your client will reject any block chain that doesn't conform to the rules as it understands them - so a massive change in rules will simply fork the blockchain into two different chains, obviously with the most popular chain being the one people use but it's entirely plausible to end up with two distinct networks that started out as Bitcoin itself.
Regarding forking the block chain, this is interesting. Would the new chain always start at zero? could it be used as a more local version of a currency? Would it have an independent supply rate?
If you have some reference to point me to I would appreciate that!
Go to the developer forum and you'll see some guy is offering Bitcoins to developers to help him start up his own blockchain and corresponding client, and miners to help him mine the new currency (he calls it Groupcoin). So anyone can do it, but just because you can doesn't mean they'll be worth anything. Bitcoin is worth something because it has a large number of people on board. People have a hard time understanding this. I think the Groupcoin founder believes he's going to get rich and spread wealth to all the developers who help him (and some are). But before you invest your hard-earned Bitcoins in Groupcoins, remember that Bitcoin is worth something because of the number of people who use it, not because of some secure P2P accounting system, brilliant though it may be.
This is exactly what will occur to bitcoins, Once the limit is reach the amount of users will drop, The places of which accept bitcoins will also decrease, And the value will drop.