And now and then you can get lucky.
I switched from 3 rail to 2 rail a few years ago. Sold about two dozen engines. BUT, I have two 3 rail Sunset UP Mikes that I really didn't want to sell and wanted to convert to 2 rail.
Wa La! I got lucky at O Scale West by knowing a gent that many of you know who was at Sunset for a long time. He had complete 2 rail driver sets, pilot and trailing truck wheel sets and even the drawbar and pin stashed away from his days at Sunset. Gave me a fair deal. So, it is practical in some instances.
But, having said that. I remain glad that I mostly just bit the bullet and sold a couple of dozen 3 rail engines and got on with life. Never looked back. Wish I had done it sooner.