I'm running out of disk space - how do I expire existing media?

I have an 11TB drive for our internal backups.  I have 6 days of storage space left.  Changing the Keep Until dates on the jobs doesn't seem to change the expiration date on the existing backup sets like changing expiration dates on media sets used to.  

How do I propagate these changes across 11TB of data - it's a years worth of backups!!!!

I must say that BE 2012 has to be one of the most poorly designed pieces of software I've ever been forced to deal with....