This is a really interesting question, because what makes bitcoin appealling versus regular old paper money, also makes it vulnerable to obsolescence.
Because everyone using Bitcoin is using them on a computer, connected to the internet, a competitive P2P currency can be downloaded by everyone very quickly. The barriers to entry are very low.
I don't think the primary threat to btc1 comes from a technically superior btc2 though. I think the threat comes from a more widely-accepted btc2. If Google, Facebook, Zynga, eBay or some other middling to large internet player comes out with their own fork, it could instantly become the defacto standard.
People won't adopt btc2 because they see the brilliance of a p2p cryptocurrency, they will adopt btc2 because they want to buy a tractor on FarmVille or some cookware on eBay. FarmVille has 50 million users. How many people are members of the btc1 universe? Twenty thousand? Even if btc2 is technically identical to btc1, the miniscule btc1 economy will not be even a speedbump in the way of btc2.
What makes bitcoin appealing is exactly what makes its mainstream adoption (by corporations) unlikely. Farmville, Facebook, and eBay are all more interested in staying on the right side of the law. Why do you think Zynga doesn't let users cash out their poker chips?
Another reason why any btc2 is unlikely to displace bitcoin is because the first-to-market advantage in any online business these days so huge. Users will especially be resistant if they lose their capital. Think about how and why myspace users abandoned it for facebook, where the capital at play was social (friends and investment in profiles).
Any btc2 will need to be traded against bitcoin, and migration would look like a not-so-gradual increase in price of btc2 relative to bitcoin.
