Yes, I have a 1988 420sel. I removed the plastic sills first to clean them and to ensure that the new sills were attached well. They aren't hard to remove. Just pull up on the edge hanging out of the car. If you keep pulling the other clips holding it on will release. I have never broken one, so I don't think they are very fragile, but you might want to be careful just in case.
BTW - Do all of your sills light up, or just the front ones?