I had a friend who adopted a dog and happened to know his previous name. She liked the name and decided to keep it, but every time she called the dog's name he'd cringe a little.
She tried to desensetize it, but after a few days she decided to just change the name.
Luna's name was Luna before I got her, but I liked the name and thought it suited her so I kept it.
Personally, now I'd probably rarely keep the former name. I do like to know what it is, though, it's interesting to me. But the way I see it, new name = new life.