I'm sure I was eating more than 400 a day when I was a month out, but I doubt I was at 800 at that point. I know I ate in the 600-800 range most of that first year, but I can't remember when I started that - maybe after about two months? Maybe just go up by 100 or so calories at a time? You'll almost certainly lose weight at 800 kcal (as I said, a lot of us eat that for much of the first year), but going from 400 to 800 practically overnight would be pretty drastic.