Sent folder empty following update. How best to restore from backup without losing more recent emails?

Following an eM update at the beginning of December almost all the emails in my sent folder had disappeared (from my pop3 account), more than six year’s worth. I do not have any archiving or auto backup operating but I have an old manual backup from July. Assuming the sent emails are not directly retrievable, how can I best use the old backup to restore most of the sent emails without losing the last two weeks worth of new stuff?