Unless you are using a lesser heart with a +1,2,3,20 after it, you cannot switch classes. This is not a TR. The +(x) determines how many levels you can change that class as well. So if you are a level 12 sorc, wanting to change into a level 12 warlock, you need a +20 lesser heart.
You can get to 20 and do a true reincarnation and start over with a past life feat however, but you need a heart of wood or heart of blood to do so. Which you can buy or craft from epic tokens.