i have replaced one of my arches on my mk3 before and they are easy to take off. As you say they are simply riveted onto the bodywork and slot into the front bumper (as I recall), so all you have to do is drill the rivets out and hey presto off they come!! Once they are off you will probably need to do a bit of polishing to remove the dirt line which is likely to have formed betweeen the wing and the arch, but doubt it will be that bad.