IMO Jourdan Lewis is an example of a guy who could make the transition.
Everytime this convo comes up about a CB everyone always says "not anyone can do that, it won't work" but the truth is that some corners can indeed make the transition and I think Lewis might be one.
It's also worth noting that "making the transition" isn't necessarily a yes/no thing. For example, no one was super satisfied with Byron Jones at safety and everyone likes to mock the experiment in hindsight. But at the time we had like nobody at safety and Jones' combine measurables and intelligence were promising so for all intents and purposes yes, BJ made thar transition as far as Dallas was concerned at the time.
I won't get too far into it because I'm not an expert and you can find experts discussing it on YouTube. Suffice to say that I won't guarantee Lewis can make the transition but I think he's got decent odds and I think it would be an error to not give him a chance. Imagine if you've got a just fine backup free safety on the roster and you didn't know because you were insistent it couldn't be done.
Anyway going back to the OP, you need 3 good corners in today's NFL. We have one. If our pick hits we might have two but even a good rookie CB is just an average NFL player. Diggs looked incredible for a rookie but overall he's still a middle of the pack CB league wide at his age and experience level. So no, I don't see a problem at all with picking up Lewis even if the plan is to leave him as CB4.
Personally I wouldn't even put a position on him other than "defensive back" and then just let him fill roles on plays as his skill set allows. I would actually do this with everyone in the defensive backfield because the year is 2021 and I think handwringing about "free vs strong safety" or "nickel CB vs rangy 3rd LB" or whatever are all outdated questions. So for example I literally couldn't care less about the ongoing conversation regarding whether Neal is a LB or S. He's just a cover guy with tackle skills, give him his assignment.
Curious what
@Future has to say about that last paragraph because he's usually pretty opinionated on team building philosophy