That's what we're telling our users. "Don't upgrade to iOS7 until we say so or you'll lose email functionality," or something like that.
Option 1: You tell everyone not to upgrade their iOS. You get a few people who call in and ask why, so it ties you up on your phone/email for a little bit. You patch your EMM, you tell everyone they can update, and you get a pat on the back.
Option 2: You don't tell anyone to not upgrade, they upgrade (probably not all of your users). They can't downgrade, so they're stuck without email until YOU fix the problem. So, now you have to work on a problem while handling calls/emails from angry users.