When you subscribe a target, under the Starter or Pro plan, your subscription is tied to the target's URL. This means that you can't delete that target and add a new one under the same subscription. If you do that, you'll be unsubscribing the old target and start a new subscription on the new target.
Avoid unsubscribing targets, because if you do it in the middle of your billing cycle, you'll be throwing out money.
If your application changed to a new URL, contact our support team and we'll be happy to change it for you.
Pool of Targets
When you are subscribing a pool of Targets (e.g. 5 slots), available in Premium Plan (or Enterprise plans), you're subscribing the right of having 5 active targets. You are free to delete or add new targets, as long as you don't exceed the pool size (the 5 slots).
When you delete a target from the pool, you'll be deleting its entire history of vulnerabilities, settings, etc.