Unfortunately choosing one side means that if you wanted to switch allegiances to another faction, you’d have to pay Blizzard $30 to make the switch. This is pretty expensive, especially if you have multiple characters. Now the good news is that Blizzard has since announced that they are trying out the idea of making faction changes free of charge.
This is apparently to help ease long queues in player-versus-player servers, where interest for a particular faction is said to contribute to long server queues. So in order help make the queues shorter, Blizzard is offering free faction changes. The developers will be limiting this faction change to just one server first, Barthilas, before they move on to other servers, assuming that the experiment pays off.
This free faction change service will come into effect on the 27th of May and will last for a week. This may or may not be extended, depending on how well things go. Now the good news is that if your guild is thinking about making the switch, you can. All the guild leader will have to do is pay the normal fee, and once everyone switches over within the free period, the money will be refunded.