Is it worth it to migrate off-topic questions that already have an accepted answer?
Is it worthwhile to migrate questions that already have accepted answers?
Yes, absolutely! Those questions belong to the other site, accepted or not, because that's the place where they will be useful for other users in the future.
Caveat: if the question could belong on another site, but is still on-topic for the current site, leave it be. Focus your efforts on questions that don't belong, or aren't getting answered.
With an accepted answer it means the OP has got a solution that works and doesn't need answers from a different set of people, so in that sense migrating it would serve no purpose.
Having said that it does mean that it sets a precedent for future questions on this topic, so migrating it would show where such questions should be asked in future.