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Much depends on what IOS versions, and feature sets, are being used on the two routers; and also on your actual configuration.
Often you'll need to address interface naming changes, but beyond that, it's a "it depends" kind of answer.
However, usually configuration differences are minimal. New IOS versions will often accept old(er) syntax (and convert it).
You can do what Reza suggests, which is a good approach, or you can try pushing the whole old config onto your newer router. In either case, watch for errors. Carefully review the results.
