To make it short; sim locked to a specific carrier, you can only use sim cards of that carrier on your phone.
All the other sim cards from other carriers doesn't work and gets locked out.
So if you have the same carrier you don't need to unlock it and you can just pop in your sim card and use it right away if possible.
Yeah just put in your sim card. As long as the phone is unlocked to all networks and your sim card plan is activated on the blackberry interner service then you can change to whatever blackberry device you want and it will work always.