Harper is a negative defensive OF, and outside of one big year four years ago, he's a good but not great offensive player.
He's averaged about 3-4 WAR the last three years (BR/FG), that's probably who he is going forward.
So "not a big upgrade."
Sure, there's always the possibility that he rediscovers his 2015 magic, but after three years, the odds are pretty slim, he's had plenty of time to adjust to pitchers adjusting to him.