I think you'll be fine. You will put on weight (2-5lbs) simply from the fact that you will have food inside you instead of *nothing* but you'll soon be losing again.
Exante packs do contain a fair bit more carbohydrates than is allowed on Atkins early phases. If you are both eating and taking Exante packs in some combination then it could push you out of ketosis pretty easily as the two diets maintain the keto state via very different means: there are two ways to stay in ketosis, one is very low calories (doesn't matter what carbs you eat to an extent) and the other is eating protein and fat (in any quantity you want) but strictly limiting carbs, so mixing high carb exante packs with high fat and protein atkins isn't a great idea for longer than necessary in my opinion. If you do want to phase in slowly then i'd suggest keeping calories low until you are off the packs entirely, stick to the 200-300 calories per meal and maybe doing it quicker than one meal per week, i'd add in a protein meal for 2-3 days, then add in a second meal for 2-3 days and then come off the packs entirely. Even better, just drop the packs and go to Atkins full time right away (maybe eat light for the first 24 hours so your tummy can get used to having food again).
It is quite a monumental shift from eating nothing, to eating whatever you like in as much quantity as you need, and eating plenty of fat, the transition is hard because psychologically eating less and starving makes sense.
Good luck!
