Can I switch from Enterprise plan to Midsize plan

Hi,

I currently have 58 licenses on E2 Enterprise Plan.  I'd like to switch those licenses to the $15/month Midsize Business plan so I can get Office 2013.  It doesn't make sense for our small company (58 Users) to be on an Enterprise plan instead of Midsize plan.  The midsize plan didn't exist when we purchased Office365.  How can I go about doing this?

Thanks