You may get a big gain, you may not - it's really hard to say. A lot will depend on how many carbs you start eating - how much bread, pasta, rice, potato etc. If you switch to having a lot of those foods straight away, then yes, you're likely to see the scales bounce up a few pounds. It's probably best to introduce them for one meal a day to start with and gradually work your way up.
The trick to swapping to another diet is knowing that it's likely you won't lose much weight for up to a month. You might even stay the same for up to a month. That's actually quite normal and doesn't mean that your new diet isn't working (so long as you're doing it right). It just means that your body's sorting itself back out with normal eating, normal levels of water balance, etc.
Of course, most of us aren't that patient, decide that the new diet isn't working, have a panic, often a bit of a binge to go with the panic, then hurtle back to Cambridge.
But it will be working - you just have to hang on in there.
Of course, having read all this, you might decide that actually, you'd rather stay on 810 until you've lost a few more pounds.
It rather depends on how much of a hurry you're in...