Keep in mind that all networks can go down from time to time, though larger networks are less likely to be down for very long. The downside is that you are also more likely to have to deal with spammers on large networks. That isn't always a big deal and there are a variety of protection scripts, but it's something to consider.
As far as moving, all you do is connect to a different network and join the channel you want to use. If the network supports it, you can register the channel so that others don't take it over on you. For help with that, ask in the #help channel one the network you choose. The only difficulty with moving is to get your users to move with you.