If it is so incredibly difficult to go from being a CB to a safety how does it matter what round a player is drafted in? Why would a first rounder not be able to do it as well as a 2nd rounder can? Or a 3rd rounder? Or a 5th rounder? Or an UDFA? Especially if that 1st rounder has previous experience at safety.
But if you want a recent first round example, let's go with Devin McCarty. Started as a CB, and became a Pro Bowl safety.
As for your insistence the Cowboys drafted him exclusively as a CB, check out this video with Jerry, Stepehn and Jason right after Byron was drafted. It totally disproves what you are saying. It's chock full of talk about Jones's ability to play safety, and Jerry says the fact that Jones plays safety was prominent in the decision to take him. Later he says they were hoping to get a player who could play CB, but could also be an excellent safety if they were able to get Claiborne back healthy at CB. Garrett goes on to say that they feel he has the chance to be either an excellent CB or an excellent safety. He also says that while he will probably start at CB, he may end up playing safety.
https://tpc.googlesyndication.com/simgad/16241788381129674477