For me, this happened yesterday. As I logged-in to Yahoo marketing there was a notice and a new transition tab. On most of my client's there was also a large button that walks you through a wizard to transition the account.
I expected a second Yahoo campaign to appear in adCenter, what I got was a duplicate set of ad groups rolled in with the first.
At the end of the process was a note, to continue to operate my campaigns on Yahoo, until I got an email notice that the import was completed (I had one this morning)
I have asked, but I still don't know if there will be any separate tracking (of Yahoo and Bing). They said there would only be one bid price, but it looks like that is also translated into only one campaign.
Looks like the whole process assumes you did't arealldy have cmapigns running on Bing. So it appears I need to go sort out the dups, and delete all the duplcate campaigns out of AdCenter.