My son's partner has told him if he doesn't marry her, she will change their daughter's surname from his to hers. Is this reason acceptable? How long would it take and does it cost.
as far as I am aware the only person who can legally change a child's name is the father. However it is easily possible for the child to be known by any surname but this would not be for the purposes of any legal documents.
A child's legal name can easily be changed by Deed Poll providing everyone with parental responsibility for the child consents to the name change. If the child is 16 years of age or over (or approaching their 16th birthday), they must apply for their own Deed Poll for which parental consent is not required.