If you are a Starter Plan user, you can upgrade to Pro Plan in your subscription page. You only need to pay for for proration and can start to use the Pro Plan immediately.
To put this into perspective, assuming:
You are on Starter Plan, which costs $9 per month
You want to upgrade it to a $38 two pack Pro plan
You have already used the current plan for 20 days
When you upgrade to Pro Plan, you only need to pay ($38-$9) * (30-20) / 30 = $9.6 to upgrade.
Once you've upgraded, here is what will happen next:
Your plan will be changed to Pro Plan immediately, and all your current credits will be switched to the Pro Plan credits
You will immediately get 300 minutes *2 * (30-20) / 30 = 200 minutes of extra credits. These are your unused Pro Plan credits in this payment cycle
You can use Pro features on all your projects (including history projects)
Please note that the watermark on history projects generated before your paid plan purchase will not be removed.