Q: I changed tiers of my premium subscription. Why did I get charged right away?
A: This is a "feature" of recurring subscriptions on PayPal. It's a little confusing, but allow me to explain.
Say, for example, you have a recurring membership with Vimm that bills on the 1st of each month. On June 21st, you decide that you want to jump up from a Tier 1 to a Tier 2 membership. So you upgrade your account on the 21st. You pay for a Tier 2 membership right then and there, but you stay Tier 1 until July 1st, when your membership bumps up to Tier 2. Also, you don't get billed on July 1st.
Basically, you pay for the next month a wee bit early, and then you don't get billed for it during your normal billing cycle. This is one of those quirks of PayPal that we cannot really get around.