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 appears that is also translating to only one campaign.
Looks like the transition process assumes you didn't already have campaigns running on Bing. It now appears I need to delete all the duplicate campaigns out of AdCenter.
I agree with others here, it should have been very simple for advertisers who merely want to proceed with their existing adCenter accounts.
At the very least a note in the AdCenter account saying "Your ads are now running on Yahoo" would be nice. It is really not clear.