Goal: You own a SEOPress PRO subscription limited to 1-site or 5-sites and you want to upgrade to unlimited.
The first step is to sign in to your customer area.
Then, go to My License Keys / Subscriptions page.
From the License Details column, click View Upgrades link.
Click Upgrade license link to purchase the unlimited sites license with your current license key.
When you upgrade, the amount you paid for your current license is credited against the price of the new one. You only pay the difference.
Do the checkout.
Et voilà!