In a few episodes before, I believe mainly in Season 3, Aunt Viv, Will's Mother, and the other sisters have always been called the "Smith" sisters.

Yet in this episode, Will's Father says he's Lou Smith.

I'm assuming they figured the father should have the same last name as the son, but it's kinda confusing, as it wouldn't make sense that the other sisters all took his name.

Then again, Smith is a common name, so I guess both parents could have been Smith even before marriage.