Once research into your family's past progresses to before 1900, your ancestor's last name might not be as "fixed" as we think of last names being today. In certain regions of Europe last names changed from one generation to another or were tied to the property on which an ancestor lived (particularly in certain rural areas).
Don't assume that your ancestor's name was "fixed."
And don't assume that it changed either.
Learn about your ancestor's country of origin and determine what the common practices were in that region.