The amount it pro-rated. Once you go premium you will have a listing in the membership pages that shows how much is available for upgrade.
For example, I just renewed a few months ago so I have a $60 credit towards upgrades; next month it will be less. Of course, I'm already at diamond, so that doens't mean anything to me. But for you, you would pay less than the full rate if you went from gold to platinum, but the amount will depend on how long it takes for you to upgrade.
Hi, just a quick question, if I upgrade to a gold membership (£18) and then later I want to upgrade further to a platinum one (£30), do I pay the difference (£12) or the full £30?