This is more a long comment than an answer.
There are two questions here, one related to the upgrade and another related to the
About the upgrade. There are several surnames that are a combination of both family names, where the second one is the one that carries the tradition (Ruiz Tagle, Pérez Cotapos, García Huidobro), in those cases the second surname is important, while the first one is quite common. So, by using a combination you can create a new surname that preserves the lineage. You can even find people with four surnames, because both family names are combined, for example, a name could be something like "María de los Ángeles Pérez Cotapos García Huidobro", this is not a joke, it might happen. In some cases, the second name of a person might be a surname.
Since people from aristocracy is well versed in the genealogy of their kind, is quite difficult to change your surnames in order to get an upgrade to reach them, because aristocrats know quite well who belongs to them.
An example of the importance that aristocracy gives to their genealogy, you can find webpages like this, where some combined surnames already existed on XVI century.
Guessing, I will say that less than 0.3% of people has combined surnames. All of them belong to the upper social class. An example of combination starts with family Garcia Huidobro on 1697.
Since often people did not have a known father, it was common to double the family name from the mother (González González) or even use both family names from the mother. Only when your father recognized his children, they had the right to use his family name.
An interesting example is Bernardo O'Higgins (link in spanish), bastard son of the Viceroy of Peru (Ambrosio O'Higgins), who did not give his surname to his son, hence he was known as Bernardo Riquelme, where Riquelme was his mother surname. Only when he demanded his dad in the court he got the right to use his father surname.
Only people from aristocracy is interested in their family names, poor people does not pay too much attention to their lineage. Actually, poor people often use alias instead of surnames, hence, fiction comes to the name, not for the surname.
I do not have evidence of fictitious surnames.