When you upgrade your Store subscription, the new level goes into effect immediately. Billing for the month is adjusted; it is pro-rated for the number of days that you had each level.
You immediately get all of the benefits of the new level, including the full number of free insertions regardless of how many days are left in the month. So if you use up the free insertions for your current level and then upgrade, you get that many extra free insertions just for the calendar month in which you upgraded.
If your store subscription is billed at the discounted annual rate, the calendar re-sets; you are committed to a full 12 months at the new level.
Here's the document with the full terms of service for store subscriptions:
http://pages.ebay.com/stores/subscriptionterms.html