They didn't just change his lips and hairline, but they furrowed his eyebrow. They made him angry. Its obvious someone thought, well if he has the lightsaber he should be in a 'Battle mode' Unlike the prototype which had more of a docile expression. However I feel it did a much better job of capturing the personality of the character. Although hes surrounded by violence, hes not a violent person. He was very conflicted. Finn was torn between his loyalty to the First Order and his loyalty to Rey. When he fights TR-8R his expression is uncertain, not only because he was facing a former ally but also because it was his first time wielding the lightsabThat er.
The prototype sculpt had a look of contemplation which reminded me of a Jedi, being calm in the midst of battle, focused on his task and not allowing emotion to overwhelm him. Whereas the final product has a look of disdain, almost rage. Which I don't associate with this character.