Is your nephew very young? If he is young and might be upset about it, then I would just change their names but not tell him! You could still use their old names if he was around.
If he is older then I'm sure he won't mind. New start, new name.
I always change my animal's names, I just don't like them usually!
My Moxie was called Marty by his owners. I sent the family an email to update them a while after getting him and said I changed his name and they didn't mind at all.
And Meeko was called Marco by the ISPCA inspector, but my ex boyfriend's name is Mark, and I didn't really like the name Marco either, so I changed it, but kept it similar because he seemed to respond to it!