I read that later UNO3s have SoftwareSerial on their chip.
You have misunderstood or misremembered.
UNOs do not have SoftwareSerial 'on their chip'. SoftwareSerial is one of many libraries contained in the Arduino IDE. In recent versions of the IDE it is a standard library; in older versions it was an external library which would have to be downloaded and installed separately; perhaps that's what you're thinking of.
The SoftwareSerial library is provided in source code form. and if you want to change the behaviour or change any configuration constants you could edit the source files in your installed copy. Any sketches you built subsequently based on your modified copy of the library would include your changes.