Originally Posted by pdlamb

I wouldn't say it's impossible to reshape chainrings, just highly impractical. Consider you might want to start filing off the old teeth. You could conceivably then file new teeth in the old ring (assuming there's enough material around the outside). So you could take your old 48 tooth ring and file it into a "new" 39 ring.

Of course, even if you did it perfectly the first time, you'd probably spend more on files than on a new chainring. But a bull-headed person with too much time on his hands might be able to do it!