buzz69 wrote: If I make this change as suggested will it affect all users of this tree?
No, existing data is never altered by changing a setting on the control panel. It will only affect persons that are created in webtrees after the change.
buzz69 wrote: Can I make this change to an individual users profile?
Not automatically. Don't tell me you did create dozens of people with the wrong surname. You have to check and amend them all. Just click on the pencil icon next to the name. Maybe it is easier to delete these persons and create them again.
But you say "user profile". For webtrees this is something different than the recorded genealogical data of an individual. To me "user profile" is the data recorded of a living person that uses your website and has registered and given the role of member or editor.