Does Restoring an old backup destroy what is currently in the client (IMAP)

I have been trying Archiving, but for some reason I have lost some of my latest emails.  Before trying the Archiving I did a backup.  Now, when I go to do a restore I get a scary message warning me about possible lost settings and emails.

So, if I do restore from the old backup, will the existing emails and attachments be lost, or will the backup be added to them?