We don't cancel subscriptions. PayPal does that if it is unable to charge you 3 times.
You can select your payment source when you are starting the subscription. Even for an existing subscription, if you look at its details in PayPal, you will see that there is a link which will let you change the payment source.
As for starting the subscription, to avoid double payment, you should subscribe on or around the date till which your account is already paid upto. This date will be given in E-junkie Seller Admin > View Profile.