Yes, your body will immediately gain weight once you’re off keto, approx 4.5kgs, depends on the person and how much muscle they have. Since the body prefers carbs, it will replenish glycogen stores immediately. This is where the term “water weight” comes in since glycogen is approx 75% water. So for every 250g of glucose your body wishes to store, it will weight 1kg due to the water.
Also, Not sure what situation your body is in, but for most of us, it took decades to derange our metabolism. Richard Morris on here talked about how our fat cells become insulin resistant and damaged over time due to chronically high insulin levels. Fat cells normally live around 10 years before they die and new ones are formed. So to gain their insulin sensitivity back, it would take 10 years to swap out all your old fat cells with new fat cells that have never seen insulin levels sustained at a high level. Once that happens, your body would probably act like it should when processing carbohydrates.