That story you posted definitely sounds suspect because the person says "I believe" which sounds like they aren't even certain if something changed or they switched out. From what I've read people at least know intuitively they switched out. I'm sure it's likely confirmable if someone had a picture of a before because someone could scan the energies and would likely notice a difference that isn't accounted for just through the person changing as we all do in normal life.
I'm not entirely sure switching to being a meat eater from vege is a reason though there is a walk-in that Alba Weinmann did a QHHT session with who actually cleaned up his diet after walking in like eating more vege/raw vegetables. Here's a link to that video where he confirms it in the session. Though I will say there was a book on walk-ins I read as a kid and I remember pretty much all the walk-ins improved the life of the person as many of the people who left were kind of in some negative condition one way or another. In pretty much every story the walk-in completely changed the direction of where the life was originally headed.
I'm also not entirely sure the break from friends/family would be an indication of possession just because anytime someone changes dramatically like giving up alcohol, becoming more spiritual it can cause such a divergence energetically that their frequency may no longer be compatible with those they were around before. I've seen a lot of people here in Soulnexus talk about the challenges spiritual growth places on relationships which it does and I can attest to that.
Another reason I think this story is suspect is because of a story I read once in the 1st edition of True Singaporean Ghost Stories. So in Asian culture it is customary for our equivalent of shamans to burn paper offerings and form a trail to guide the spirit to the afterlife. We burn offerings in a lot of ways including paper moneys and such. Anyhow this person came home and felt like they had eyes glued to the back of their head. They also suddenly could no longer keep any food down besides vegetables and fruits, everything else like meat would be vomited up almost immediately. They couldn't sleep because they felt like they had eyes on the back of their head and it kept them up at night. Finally their family called in a medicine man because well, they were at their wit's end. Turns out the guy had stepped on a trail of papers burned to guide a recently deceased soul and the soul sensing an interruption in the trail confusingly attached itself to him. The guy was instructed to burn some offerings and I think relocate the trail by tracing his steps back that day the "eye" issue started and it worked. He later asked the medicine man why he could not eat meat and the medicine man simply replied, "well the soul was vegetarian." LMAO
Some of the symptoms this guy reported are similar in that the dietary change occurred. Both possession and walk-in could account for this. Though it's funny that in this instance the confused soul was vegetarian and the guy stuck with it was not.
Another thing that comes to mind are the symptoms that come when people get organ transplants. This person had reconstructive surgery and I'm not sure if something was transplanted into them from someone else like perhaps a skin graft or etc from a previous body. I suspect though I've never directly gotten to read a situation like this is all objects including our bodies would like contain residual energies of our habits and patterns we had when we were alive, perhaps even soul fragments as the possibilities are endless. There are cases where people get organ transplants and end up immediately acquiring new habits they never had or craving strange things they never used to want. It isn't "possession" in the sense of a conscious entity in the space of another but are foreign energies that are in the space of another. I could see it totally possible that someone would acquire soul fragments from someone or energies ... that could be loosely defined as a sort of "possession" as well.