Unlocking Blackberry Bold 9700 and using the original carrier
You can unlock Blackberry phones by using remote unlocking services.
When a Blackberry is sold by a carrier it's branded to the original carrier and can be used only with sim cards from the same carrier.
the unlock code is used to disable the SIM lock that blocks the use of your Blackberry Bold 9700 with sim cards from other carriers.
When you unlock your Blackberry Bold 9700 you actually not doing anything to the settings of the older carrier, you are just disabling the block that keeps you pone locked to one carrier.
You will be able to use unlocked Blackberry Bold 9700 with your original carrier and with other carriers, when a sim card from another carrier is inserted the new carrier is used and you can activate the data plan by going to the device options and selecting host routing table "register now" option. it will activate the data use on your device, the calls / sms will work immediately after the sim card is inserted.
Same thing about switching back.