What happens when I upgrade/downgrade my subscription?

When you change your subscription two things happen.

1. Your current subscription is set to end at the end of it's current billing period. In other words, it's set so that you will not be billed again for it.
2. Your new subscription will be billed and started immediately.

For example,

Let's say you have a Basic Plan. You want to upgrade to Pro.  You click Subscribe to Pro. Your Basic Plan is cancelled and the Pro plan is immediately applied.

The new submission credits from the newly purchased Pro plan are available immediately.