Yuck, how petty of your ex (I’m assuming) to create litigation over something like this. It’s totall unnecessary and clearly about control.
It has nothing to do with it being in her best interests, best for a child to know her father, be aware of her full identity. Children in this country don’t take both parents surnames automatically.
It’s just a convention to name after the father so the whole family has the same surname, which makes sense in a family that is still living together as a unit.
If she is living with you then it makes more sense for her to have your surname for her to identify with the parent who cares for her most. Also it will make it way for you wen dealing with all administrative things e.g. schooling, healthcare, travel.
My daughter has my ex’s name and it’s really impractical for her to have his name not mine when I’m the one who deals with all the practical stuff for her and she lives at the same address as me.
I’m remarried and I have taken my husband’s name so we will all (with the exception of my DD) have the same family name. It’s a shame that my DD is going to have a different surname from her siblings. I have a different surname to my half-siblings and I hated it growing up because it really separates the family unit.
I’m sure you don’t want to change your surname to that of your ex so it’s right that your daughter should keep yours.
It’s on him to explain why such a change is necessary at all when they child has already had her name for months.