You can't change the class of your character once it is created. Each free account comes with three character slots so you can choose a different class for each character. The only thing you can change for your character on the fly is their crafting abilities.

There is something called Monster Play in LOTRO, but it only is available with the VIP accounts. Free accounts don't get it. I recently added the Mines of Moria expansion and got a free 30 days as a VIP. The monster play is pretty fun, but I haven't gotten too into it since I'm focusing on using my VIP advantages for my usual characters for now.