Eat meat on occasion, and treat yourself to the good stuff, on occasion, like on holidays. And eat the fat, too. That satiates your appetite quickly.
I realise that eating meat is a luxury and consume it as something special.
When the aliens come I don't think they'll be too surprised at seeing humans committing violence against each other through wars, but will marvel at the extremely inefficient way we use animals for food.
Raising the price of meat to the point where one can only afford to eat it once a week is more realistic and attainable in five years. Then from there, consumers may realise they don't want to pay for the luxury and stop eating meat altogether.
Realise that people's identities are tied to their diets. To ask them to change their culture requires either a Year Zero type change, or more subtle nudging.