Expiry emails- list of users

An expiry email just went out to people on March 31. How do I obtain a list of those users who received the email?

On an unrelated note, how do I find a list of the users who have upgraded their subscriptions at any point?

If it's a CBSubs Mailer email then within CBSubs > Mailer > Sent Messages should be a history of sent emails. For the Presentation tab or CBSubs Emails usages there is no log of the emails sent besides maybe your hosts mailer log it self.

On an unrelated note, how do I find a list of the users who have upgraded their subscriptions at any point?

Within CBSubs > Subscriptions you can filter on various subscription statuses.