This is a funny thing and related to real climate change over billions of years. A few billion years ago our atmosphere started out as carbon dioxide and methane at 20% with the usual nitrogen. Our temperature was shirt-sleeve as always. Bacteria lived, eating the methane.
But, there was this great opportunity with the co2. A molecule formed from mutation, and could trap co2 and do something with it, by hitting it with a photon. The method of photosynthesis evolved in high co2 and has never changed since.
These early bacteria, and then plants had a heyday. They chugged away eating co2, and leaving oxygen as pollution. Why did we settle at 20% o2? Because the plants were stewing in their own juice. They couldn't handle that o2 and photosynthesis started to get screwed up. The molecule was picking up o2 as easily as co2.
The plants handled this by dying. But they evolved a kludge tack-on mechanism that cleansed o2, through respiration. With this they could probably go up another few percent of o2.
That respiration mechanism was picked up by animals who could live with o2, and eat plants. Yeah!
But now scientists can fix the kludge and increase production by 40%. Can we see a problem with that? We can drive down co2 and increase o2 percentage. No unforeseen consequences here! :)