Can I upgrade my account by only paying for the price difference?

Currently we do not support upgrading account by only paying for the price difference between the new plan and the current plan.

For example, you subscribed to Basic plan on July 8th, and intended to upgrade to Pro on July 20th. If you choose "Bill immediately" option, then you need to pay $19.9 on July 20th and receive 500 credits. And you will be billed $19.9 every month starting from your next billing date, August 8th.

However, your credits will roll over. The credits unused from July 20th to August 8th will roll over till next month, so you can still use them as usual.