I've added to either my Pay-as-you-go (formally On-Demand spending) or reserved quota by upgrading my subscription but I am still unable to view any of the events that were dropped during the time period I was over quota.

In this case it is not possible to access the previously dropped events; to clarify once you have reached your event quota limit we drop all events so this information is not stored by Sentry in any way. Once you increase your event quota, we’ll begin accepting and storing event data from that point onward.