If you follow the Excel Diet you won't gain the weight back, it's impossible.
The Excel Diet works because what you do never changes. Many diets celebrate that you've reached your goal, but not us. If you gain the weight back you suffered for no reason.
Don't diet if you think you'll gain the weight back
Now the key with the Excel Diet that you always have a daily goal. You always have to weigh yourself each day no matter what. You always have to modify what you do based on where you stand. The spreadsheet tells you what to do.
If you're willing to do these steps then you'll lose the weight and keep it off. If not you won't, it's that simple.
If your choice is to lose weight and keep it off, then continue. If not then go to a diet that really just wants you're cash.