I think you guys are being a little hard on the OP. He appeared to be of the view that there was a legal requirement that they take the seats as assigned. In this crazy industry anything is possible!
Try reading an airlines terms and conditions (which everyone SHOULD do) and you will see where he got the idea from.
He now knows that there is no legal requirement and, within his allocation, he can do what he wants. Swapping seats with other passengers is also legal, but a bit more complicated.
Also, with paid seat allocation, you generally can get what you want - if you do it far in enough in advance. If it is a concern you can pay. You will be happy, the cabin crew will be happy and your fellow passengers will be happy.