You don't have to update the IMEI, since the Desire Z uses a SIM card. You just switch your card. If you don't have a SIM card right now (I don't know much about the Impact) then you pay an activation fee to do it.
You won't be able to get the phone without a data plan. Depending on how far into your contract you are, Bell might offer some kind of incentive on the Desire (unlikely since it's so new) so odds are you will be paying the $499.95 price tag.