You will be able to upgrade, but of course we don't know the pricing yet. They may operate like with Snow Leopard and state that the stand-alone update is only from Snow Leopard to Lion, and Leopard users would have to purchase the Mac Box Set, but I think more likely they'll just have a stand-alone update that anyone can purchase. We won't know for sure until it's released, but we do know that you won't have to purchase Snow Leopard in order to get Lion. That's never been the case with any Apple OS X update.
jW