If you already have AdBlock, you don't need to uninstall it when you install AdBlock Premium. We generally recommend using just one ad blocker at a time. However, it won't hurt anything if you decide to keep both.
By the way, AdBlock Premium is not an upgrade to AdBlock. They are two completely separate extensions. Your filter list subscriptions, custom filters, and other settings in AdBlock don't carry over to AdBlock Premium. Learn more about the differences between AdBlock, AdBlock Premium, and AdBlock Premium Gold.