As this is your first post, welcome to the Pure Planet community! When you post here you are not talking directly to Pure Planet but to your fellow customers. They are usually a helpful bunch!
The lower summer payments and higher winter payments are a key built-in feature of the Pure Planet offer. Many of us find it quite a helpful way of avoiding an over large credit during the summer.
I can't fully understand your concern, because the system is intended to keep you in credit roughly equivalent to one month's payment (because you are paying in advance). If you have built up a significant debt then something is not quite right - you could be using more energy than last year, and/or your payments need adjustment. Pure Planet review your account every six months and suggest an adjustment to your direct debit payments if necessary. You can alsomake a one-off payment if you want to.
Finally, I wouldn't worry about the account balance if you want to switch - after you have left, Pure Planet will send you a final bill (keep your direct debit open). I've never heard of an account that has included regularly paid direct debit payments ever being blocked for a switch.
Hope that helps. Post here again if not...