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!