@ednabaggins – hello! I don’t think there’s really a formal transition. It’s probably just a marketing ploy to get you to sign up for a program. If there is a transition, it’s certainly not necessary. As the book says:

“Once you have kick-started your weight loss with the Very Fast 800 you are on a roll. It is entirely up to you when you move on to Stage 2, the New 5:2 – some of you will find a couple of weeks’ rapid weight loss enough; others will want to stay on Stage 1 for longer, depending on your goals. But either way your hunger levels and your cravings should be much reduced. You should also find you are feeling more energetic, lighter and brighter. People will have begun to comment on how well you are looking. Switching over is straightforward and can be done at any point that suits you. For the 800-calorie fast days, you continue to use the low-calorie recipes in this book (or, if you prefer, a mix of meal replacement shakes and real food). On the other days, you can eat normally, but healthily, i.e. sticking to a Med-style diet, low in carbs and refined sugars, without counting calories.”

So you can move to 4:3 or 5:2 whenever you feel ready.