Welcome to the Pure Planet community! We're fellow customers who do what we can to help.
The whole way Pure Planet works is that you pay in advance for your energy, so a first direct debit is taken the day your switch is completed. This obviously means there can be an overlap with your previous supplier until both companies receive and accept the agreed reading for your switch point - which can take up to six weeks.
If you are in credit with your previous supplier, you could contact them and ask for your direct debit to be reduced to a nominal sum pending the final account.
Once you are with Pure Planet, you can change your direct debit date as follows: If you go to the app, click on the three bars at top right, go to Balance and Payments, there you will find details of your payments and date of next payment, along with a facility to change the date.
Thus when you have made your first payment, wait until you are less than a month away from the date you want to pay, make the change, and you should find that you will go 6-7 weeks between your first two payments.
If I've got that wrong someone will post here sharpish!
Hope that helps