To replace them, you remove the manifold, then slide the burners out through the cook box. DO NOT remove the screws holding down the front and back burner tubes on the left side of the cook box. They are NOT meant to be removed. They are simply place holders. Then, just slide in the new ones and replace the manifold.
It is kind of confusing to do it for the first time, but if you check youtube, I am sure there are videos there.
Also, if you really want to use genuine Weber burners, that is fine, but you can pick up a set of aftermarket ones for much less and they work fine. I use them on all my rehabs and even my personal grill. They are generally around $20 a set. Here is a good set. Just make sure you click the 30% off coupon.