Carbs put glycerin in your muscles and liver, and that can lead to quick gains on the scale. It’s not inherently fat. You will gain some lbs once you introduce carbs, but that doesn’t mean it’ll continue to go up exponentially. There’s a lot of info on youtube about how to ease off of a keto diet without gaining a ton of weight.
Edit: You will go through a period of digestive discomfort when you switch. If you’ve been eating only keto/carnivore for a long time, your body begins to do away with its ability to process carbs because it hasn’t had to. Doesn’t mean it can’t re-learn that function, but because its main source has been ketones for so long, it’ll be a bit of a transition period, just like it was when you first went from carbs to ketones.
Also, your gut bacteria would have totally changed. The stuff needed to break down plants will have died off and you’ll only have the stuff that eats meat in your gut. So you’ll definitely wanna get some high quality probiotics and digestive enzymes and make sure you transition slowly.