Already BES Express 5.0 SP2? If not read on...
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Note: BlackBerry Enterprise Server Express 5.0 SP1 does not support the use of the BlackBerry Enterprise Transporter tool. BlackBerry Enterprise Server Express 5.0 SP2 does support the use the BlackBerry Enterprise Transporter tool.
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KB22801-How to upgrade from BlackBerry Professional Software to BlackBerry Enterprise Server Express 5.0 SP1
KB21126-How to move users to BlackBerry Enterprise Server Express 5.0 SP1