You have to login to your account on diggernaut.com and proceed to Billing > Subscription section, on the left side you will see cancellation form. Once you cancel your subscription, you will still can use your resources during current billing period. And at the start of next billing period your account will be switched to the Free plan.
Definitely, yes. Once your payment is approves, system will add more page requests to your account and extend other resources according to your new subscription plan. You will be able to use all benefits almost immediately.
If you are upgrading your subscription plan, it will become active immediately.
If you are downgrading your subscription plan, your new subscription plan will become active on start of next billing cycle. Until that date you will stay on your current billing plan.