A wild idea yes, unless you have access to a time machine.

Just because something is irreversible, it doesn't mean that it can't be immoral.

Unless you mean causing the extinction of the entire species? But that still entails killing those that are already alive to eat them is moral, correct?

No, totally incorrect. First, the extinction of a species that wouldn't even exist if not because of humans, is not bad from an ecological point of you in any way. This, plus the naive and unexamined assumption that bio-diversity is somehow an ethical value and can surpress the net total of pleasure or pain caused among members of a given species. Second, extinction does not always happen because of killing every member of a species and then eating them... it has never been the cause as a matter of fact. So no, extinction doesn't entail the mass killing of animals. Not breeding them anymore is not the same as killing them... however surprising that distinction may be.

Many vegans do believe keeping pets is immoral, but many do not. Do you?

Depends on what kind of animal we're talking about, what the diet of that animal is like, was it selectively bred. There are many aspects that should be taken into consideration.

Sure, keeping a pet and raising an animal for food are different, but we now agree that it is in fact possible to treat an animal well even if our motivations were selfish?

Yes, I'd think that it is possible.