You can indeed use the same server / os installation, however you'll need to remove / uninstall BES for Domino, and then install BES for Exchange. You won't need to wipe the OS, but you cannot have both installed at the same time.

You could issue the wipe command from the servers, however that will delete all data from the handhelds and you might not want to do that. Given that every handheld will need to reactivate I think you're best having the users initiate a wipe so they can back anything up that they might have on the device.

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