Not sure if it needs saying but I've namechanged for this one.
I have a 4 year old stepson who doesn't know a life without me as I've been with his dad for 3.5 years. I have a good relationship with his mother, we get on very well, and we have hardly any conflict. I wouldn't say we're friends though, most communication goes via my SO and is generally purely practical. We don't do joint birthdays or anything similar.
Sometimes she deals with some jealousy because my stepson is very attached to me as I've been a mother figure in his life pretty much since birth, but we communicate and work through it which takes compromising from both sides. We have 50/50 custody.
Now the one thing that is causing a pretty big issue is that I am getting married next month, and was planning on changing my surname after marriage. My stepson's surname is double-barrelled as she and my SO were never married. She sent me a message saying she has been trying to work through it, but she can't accept the fact that I will have the same surname as her son. Because of how attached he is to me, when I am out with him and my SO, people will always assume he is my son, and having the same surname as him means that we will be 'erasing her'.
I get (to an extent) where she is coming from, but I wouldn't even have the same surname as him exactly, as obviously I won't be double-barrelling my name to hers haha. I don't know how to navigate this, as I don't want to upset her, our sour the excellent relationship that we have, but it's also just important for me to change my name to my husband's.
Does anyone have any experience with this? Or any ideas on what I can say to her? Or should I just pick my battles and not change my name until she's had time to come to terms??