You can change servers for an already created character for a fee of $25. This feature lets you change all and/or any of your characters to any server, but you have to move them all at the same time.

Meaning, you can pay once to move Character A from Server A to Server B, and at the same time move Character B from server X to Server Y. But after you've confirmed the move of your characters, you have to pay again to do it, and you can't do it until three months have passed.

I assume this is partially because SE wants to make money,and partially to stop people from changing servers all the time and for no particular reason, and to stop real money traders from farming items on a low population server, and sell them on a different server.