Everything will be changed or at least adjusted. I know one of the things that won't change is probably the way you talk to NPCs - meaning that you will never see a drop-down box with available phrases that you just click on. You'll always have to type what you want to say.
But when it comes to combat, magic, crafting, training, guilds, buddy lists, movement, pets, nothing is solid. We're looking to get as many things basically working as possible before starting to think of adjustments and improvements for them, so complete redesigns aren't going to pop up for a long time still.