My sister is a vegetarian, and from what she's told me she started because of animal cruelty issues, but she kept going because (a) it's kind of a point of pride to maintain a lifestyle deliberately different from other people's, no matter how minor, and (b) after not eating meat for a while, the taste becomes strange and unpleasant. Like any 'new' food, she could reacclimatise to it if necessary, but it would require effort.

So I'm guessing that many existing vegans and vegetarians wouldn't eat lab-grown meat. It might well cut down on the number of people who become vegetarian in the first place, though.