Assuming this is an honest question and not a lame attempt at a gotcha: as more people switch to a vegan diet we will gradually stop breeding them in the first place, so there are no animals for something to "happen to."

Sort of both: OOH, mankind once again causes mass extinction for a few dozen species. OTOH, the oceans will be teeming with life.

The gotcha is mainly that one can support the welfare of animals but also be ok with their systematic extinction. I don't know which outcome is worse.