The easiest way to introduce changes to Bitcoin is almost certainly through creation of an alternative currency with a new name. This has been done already quite a number of times, such as with:
- Litecoin, probably the most successful alternative, which uses a CPU friendly mining algorithm instead of a GPU friendly one. Unfortunately this property allows botnets to mine coins more easily.
- SolidCoin, which when I last read about it was partially centalised and closed source.
- Geist geld
Litecoin seems reasonably mature and is still growing (at least, in hash rate). PPCoin is very new, and I don't understand its mechanism. I'm not sure what's happening with Freicoin. All the others listed above are essentially dead or dying as I understand it.
There is also NameCoin which is designed to provide a decentralised domain registration system.
It historically hasn't taken long for exchanges to become available for new cryptocurrencies, which allows for easy conversion from Bitcoin to the new currency. Other supporting tools like automatic payment systems tend to be developed later.
However, so far nothing has even come close to Bitcoin's popularity. If there was some amazing new feature provided by a new alt-coin that Bitcoin couldn't provide, I could imagine the switchover from Bitcoin being quite rapid. It's just that nothing has provided an adequate point of difference yet.