from https://www.extendoffice.com/documents/word/5513-word-comments-author-name-change.html

Sub ChangeCommentAuthor()
'Updated by EntendOffice 20181112
    Dim I As Long
    Dim xNewName As String
    Dim xShortName As String
    If Selection.Comments.Count = 0 Then
        MsgBox "No comments in your selection!", vbInformation, "KuTools for Word"
        Exit Sub
    End If
    xNewName = InputBox("New author name?", "KuTools for Word")
    xShortName = InputBox("New author initials?", "KuTools for Word")
    If xNewName = "" Or xShortName = "" Then
        MsgBox "The author name/initials can’t be empty.", vbInformation, "Kutools for Word"
        Exit Sub
    End If
    With Selection
        For I = 1 To .Comments.Count
            .Comments(I).Author = xNewName
            .Comments(I).Initial = xShortName
        Next I
    End With
End Sub