CB is by far the easiest position to play, especially if you played as defensive midfielder. The other way around is definitely more difficult and I don't if there is actually a current footballer that started with CB as his main position and turned into a DM successfully(not like Jones or Pepe). Does anybody come to mind?
David Luiz being a modern day example? granted hes been moved because hes defensively horrific rather than anything else.
We at United have a history of turning or trying to turn players into centre mids that have no place being anywhere near the position, we did it with Alan Smith for a while, christ that was rough viewing, even rougher when Fergie came out insisting it was working
we did it with John o'Shea to, god knows why, Giggs has been used there a lot more in recent years to and more recently its been Jones and Rooney.
Basically if any player out there who isnt a centre mid but fancies a crack at it they should join United, we're masters of it! lets not buy a midfielder lets buy a striker/defender and turn them into a midfielder now theres a challenge....im convinced Fergie took this challenge on around 10 years ago through sheer boredom and fancying a crack to see if he truly was that much of a genius.