I'm not sure if the whole body regenerates over 7 years. I think "replacement" is a better word, though. Cells are incredibly dynamic, and they're constantly replacing old worn-out proteins and molecules. It's not so much that whole cells get replaced (though that does happen with some cell types, like skin), it's that the individual components of the cell are being broken down and new ones synthesized in their place.
For an analogy, imagine repairing a house. Skin cells replace themselves entirely, which would be like taking down a whole wall and putting up a new one in it's place. Other cell types, like neurons, only replace parts of themselves, which would be like carefully replacing a wall one brick at a time. You would remove one brick, and put in a new one, remove a second brick, and replace it, and so on. Replacement in this fashion takes longer, but you still have a functioning wall during renovation, and the new wall looks the same as the old one, albeit with new bricks (i.e. allows the neuron to stay active and retain it's identity).