The whole point of the meme is contrasting tiny ineffectual lifestyle changes with actively targeting the people responsible for climate change. Changing the "meatless mondays" panel to "going vegan" ruins the meme by replacing what's supposed to be small and ineffectual with something that's actually radical.
Yes that'd be a great step in the right direction but both are examples of radical change. Even if we lived in an anarcho-communist society killing animals when we don't have to is unnecesary and cruel