It's completely possible to do a vegetarian low carb diet. It requires a little more planning, but if you are already doing meal prep, that's half the battle. There are tons of resources online, including Pinterest and Reddit. If you would prefer to stay vegetarian, there isn't any reason not to do so

As for transitioning over time, that's totally up to you. There's definitely a learning curve in the beginning as you figure out what is higher and lower in carbs, but I recommend you just commit and do it to get your body keto-adjusted and over it. You will have the carb flu for the first week or two, but once your brain adapts to burning ketones instead of carbs, you'll feel like a whole new person. You won't see the results you want going the slower route, and at least for me, those results are what made it possible for me to stick to the low carb diet. If I was just cutting out the food I loved and still not losing weight, I wouldn't have been able to stay the course. When you transition all at once, you lose a huge amount of glycogen/water weight the first week. That was a huge motivation for me to keep going.

I do a cyclical keto diet, and I have a treat meal once a week to refill some of my glycogen stores to fuel my workouts. That gives me something to look forward to, and makes it easier for me to stay motivated during the week. I've lost 21 lbs since mid-January, so having a higher carb meal each week doesn't seem to be affecting me negatively. My workouts were pretty terrible during the first couple weeks of transition, but now they are as good as ever.

The thing about soy is that it converts into a phyto-estrogen, which can negatively affect people with hormone imbalances. It's up to you if you want to find out how sensitive (or not) you are to it. I can eat a lot of dairy, and a lot of people don't do well with dairy. It totally depends on your body. I still eat tofu on occasion, but I switched from soy milk to almond. Cheese is low carb, but cow's milk is not. You have to decide what changes are important to you to be able to stay within your macros each day. I do <50g carbs, 100-ish grams of protein, and the rest fat. I eat about 1600-1700 calories a day. It's the same calories as I was eating before, but dropping my carbs from 150 a day to 50 made me start losing weight again.

It's a lot of trial and error, but it's been totally worth it for me. I can even sit in front of a basket of tortilla chips at a restaurant now and not be tempted by them. It's really the fastest and most noticeable change I've ever seen in my body, and I've been working on it for a while now.

I'm not vegetarian, but I do think a gradual transition should be fine.

I know that people say that you need to stick with this for it to work, and after a little bit it will be get better. I have to say though, I started on Monday (50-51 carbs a day) and this sucks. My workouts have been more sluggish and difficult to get through. I'm sleepy. My head hurts. I'm bloated. I haven't weighed myself, but I swear I can feel myself gaining weight. All that, and I'm still hungry, all the time.

I know stick with it. Give it at least 8 weeks before you judge.

At this point though, god is this miserable.

I know this is a little old, but a few things to keep in mind:

1. Ease up on the exercising, especially cardio. When you're running on sugar, you body switches to it pretty early in exercise. When you suddenly stop giving your body those carbs, it's going to freak out and not be able to function properly in that situation. If you do cardio for the first couple of weeks at all, make it light/low-intensity stuff.

2. Drink tons of water. Fewer carbs = less glycogen = less stored water. This means you need to drink more in general, especially while working out.

3. Don't limit sodium. Low carb diets are naturetic, meaning they prompt the kidneys to flush out sodium. This can lead to electrolyte imbalances and deficiencies. You can get away with 3,500mg or more without issues.

4. Don't restrict calories during the initial couple of weeks. The transition is hard enough as it is, don't make yourself more miserable by trying to restrict calories. In fact, track your food to make sure you get enough, and if you're hungry, eat. Even if it means going over your calorie allotment.

And no, odds are, you're not gaining weight. It probably just feels that way, because we've been conditioned to avoid fat, and it takes getting used to eating it again (and you better be eating fat, if you're restricting carbs!).