Thing is, you can't you restructure the client overnight, when the new alt-coin has something truly innovating (which would be the only reason to switch) it would require an hardfork in bitcoin, if you would want to implement it. Not something that will happen overnight.
On the other hand, switching over to the new currency (by trading) would be real easy.
Buying the new currency would be easy for pretty much everyone on this forum. However it would likely take years to build up the infrastructure and community that Bitcoin has now. I think people vastly underestimate how hard it is to build up the infrastructure and community that Bitcoin has now (Erik Voorhees has talked about this a lot recently).
On the other hand, even for a radically different new coin, I expect the Bitcoin community could integrate it within a couple months as they'd have the source code of the new coin.