[77newell]

It's perhaps just me but if I were doing the job myself I would replace them all at the same time. I found that after the first one the next ones were much quicker. You can learn a lot just by changing one. If you are like me you will quickly forget what was just learned, if there is much of a delay to changing the next one(s) I will be starting over. The bags aren't terribly expensive and my time is valuable. Also, the less often I have to crawl under that beast the better.