We went thought this 2 years ago and followed a similar process as hdawg and others pointed out. Since I'm in the minority here in that I'm not an email admin, I worked out a process to make things go as smoothly as possible.

We literally had a schedule of the day that each and every user was being migrated. At the time, our BES user count was only 120, so it was manageable. We communicated to all BES users what the process would be to reactivate each BlackBerry after their mailbox was migrated. I then scheduled with each user ahead of time to backup, wipe and reactive their devices (most however when asked, didn't want/need a backup taken). Depending on the data on the device, you might be able to get away with backing up the data now instead of waiting.

We didn't care about the OS back then as we do now, so certainly taking the opportunity to upgrade the OS is recommended. Use the JSanders method of updating the OS's on multiple devices at once if you can.Updating OS on Multiple Devices and adding 3rd Party App Install