Pick the bigger plan on the pricing section and check out with the same email you used before. Your site, address, and data stay exactly where they are — we rewire the plan around them and confirm by email, same day.
Upgrading mid-cycle is always fine; the bigger allowance applies as soon as the switch completes. If anything about your situation is unusual (custom domain mid-move, big build in progress), mention it in a note or email us first and we'll time it so nothing interrupts.