Just my opinion, but I guess if I were on another weight loss plan before SBD, I'd just start SBD from Phase 1 and go from there and not worry too much about a huge weight gain. It might take you body some time to adjust to SBD, so there might be some fluctuations, but I don't know if you would gain a lot of weight back on SBD.
A friend talked me into going on Atkins after I drifted away from SBD. I lost weight, but my body couldn't tolerate it at all. I gained back all the weight I lost when I stopped Atkins. I started eating anything that didn't make me feel sick - no diet plan.
Now here I am on SBD again, and I lost 6 pounds so far, maybe more, but I am only weighing myself every two weeks.
Give it a shot. It's a healthy way of eating that you can maintain forever, with a little planning and determination.
On the Beach for Life!