For instance, does the system recognise that I already have a previous purchase, say Tier3 and allow me to pay the difference in price to get to the higher version, say Tier 4, or do I have to pay in full for the higher version ignoring the previously purchased tier? I cannot see a way of letting it know I have a version already. If I click to buy the higher tier it shows the full price of that tier, ignoring my previous tier.