DD came to live with me full time at 3yo. We are now a permanent foster placement, so I can't legally change her name.
That being said, I have "changed" her name informally. Her birthname is a nickname for a few other names. I picked one of those names, and started calling her in casually on occasion. I think it helped that she's always been a first-name-and-middle-name-together kind of kid for whatever reason, so being called a longer version of her name didn't seem "weird". She knows the difference between her "long" name, and her "short" name. She answers to her "long" name much more consistantly.
In the adoptive groups I know, the last names are always changed, and it's pretty evenly split when it comes to first and middle name changes. It seems that the older the child, the less likely they are to totally change the name, and the younger, the more likely. JME, YMMV, of course.
--Mimi
Mom to Lala (2004), Bonus Mom to Big Sis 1 (1991) and Big Sis 2 (1992)
Grammy to Big Kindy Kid (2011), Big Pre-K Kid (2012),
Grandbaby Appendage (2014), and New Baby Grandboy (summer 2017)