Q/A: Should I Switch from Bing to Yahoo?

Answer: You shouldn't remove yahoo paid search in favor of Bing, both are working independently for the time being. The deal between Yahoo and Bing is not certain yet as they are awaiting regulatory approval and then after that it could be up to 24 months before the merging of their natural and paid search systems.

At this time, I would suggest setting up a Microsoft Adcenter account and copying your existing ad information from your Yahoo/Google accounts to Microsoft Adcenter (since you don't have an existing Microsoft Adcenter account).

Then, having all three accounts (Google, Yahoo and Adcenter) running concurrently bringing in the maximum amount of traffic. When (and if) the deal goes through, I'm sure they will provide more details about whether they plan to merge the accounts or not and how to proceed, but again, we have some time before you need to worry about that.