Can I change "she’s fairly happy" to "she’s pretty happy"?
Yes, you can interchange "pretty" and "fairly" without impacting the meaning of the sentence. Both "pretty" and "fairly" are used to mean, a little bit or not completely. I would say, though, to avoid using "pretty" in this meaning in business or formal situations and use "fairly" instead. "Pretty" is generally viewed as an informal form. Ex: The article was pretty good. Ex: The article was fairly good.