Arnold,
Great thread. Since I moved 7 years ago, I have several engines that have not left the shelves. I recently place my Railking Big Boy on the tracks and she was sluggish. She's been back on the shelf since. My thought at the time was I needed to really run her a lot again to get her to run smoothly. Unfortunately, I'm working on my second/third level and have crap piled all over the layout and the trains aren't running at all until I finish this phase.
It'll be nice when all of my track is down, and I can run every engine. Changing them out seems to be a great idea. On my last layout I kept a maintenance book to note when I oiled each engine and changed tires. That also allowed me to keep track of making sure I 'was mixing them up to keep them all running.
Mike
