Titles are for marketing and egos. No one's role will actually change. Maybe T-Rob will help the DC a little, but he would still coach DB's. DVD is fine as CB coach for a while.
This is why I think giving T-Rob a co-dc title (with someone above him as dc) might be a good idea. He can be part of the recruiting for all high-end defensive targets regardless of position, and be able to say he will be coaching them too as co-dc.