I have a case in colonial New England that is similar. Adrian Unknown married John Bland and had a son. John died and Adrian married a man named Smith. Smith died and Adrian married third, a man named Norcross.
Adrian's son sometimes used the name John Bland and sometimes used the name John Smith. If you know the a mother that re-married, you might look for the name of the previous husband.
Also, undocumented adoptions are a fairly common way of changing surnames. As an example, in the wilderness areas an orphan was adopted and raised as one of the new family.
There are usually good reasons for a surname change, you just have to find out the history. I suggest a yDNA test to see if the genetic paternal line can be determined.