I've heard people who say they were vegetarian and then went back to eating meat and did just fine. This always made me a little suspicious. I believe they probably never were really strict in sticking to their vegetarianism to begin with. They probably did a little meat or meat by-products every now and then anyway, so it was fine for them to go back to being omni with no issues. But someone who is 100% vegetarian all the time and then trying to go right back to eating meat is probably going to have some issues with it until their bodies get used to digesting animal proteins again.