If you have identical accounts/campaigns in MSN and Yahoo, is it necessary to deal with all the merging and shortening of ads, etc. so the Yahoo campaigns will merge seamlessly? Or can I just forget about my Yahoo accounts and let them die, keep my MSN accounts and campaigns, and simply check a box or something saying "run in Yahoo"?
Yes, you're better off just continuing on with your current MSN accounts. There's no reason for you to pull in an identical account and it would actually be a bad thing since you'd have to start your account history from scratch.