You may or may not have to change the motherboard when upgrading the CPU in a desktop or a notebook. It depends on if the motherboard (1) supports the CPU and (2) has the same socket as the CPU.
In the desktop world, upgrading motherboards is easy. This is not so in the notebook world, where it's next to impossible. Finding a new motherboard that supports different hardware and still fits inside your case is a losing battle.
If you're going to upgrade a notebook CPU, your motherboard must support it and have it's socket type. It is also not advised to use a CPU that has a higher TDP than the one you're replacing, simply because you cannot upgrade the cooling system of a notebook.
LOL, no no you do not always have to change the motherboard when changing out CPUs with Intels or AMDs. It depends what kind of chip you are upgrading to, i.e. different chipsets, sockets, etc...Your best bet in this regard, at least to make your own decision on the matter is to check out the CPU forum. Do a search and you'll find a wealth of information on the topic. Some like AMD, some like Intel. You can also check out the hardware performance charts on Tom's here to give you a better idea.