The answer to the original question is "you can replace them 1 at a time". That's exactly what my Lincoln Service Rep and Mechanic advised my wife to do (I was out of the country).
However, when you spend the effort to get to one of them, the remaining 3 on that side takes just another 5 minutes max to complete. Lincoln Service wanted something in the neighborhood of $400 to replace 1. The labor is in the removal of the cover, or better yet, that last bolt.
Of course, there are lots of other great reasons to just replace them all (see above).