Generally speaking you can change plans whenever you want with Fido with no fees. The only exception I can remember is some older plans (preRogers or with SAF, etc) there was an admin fee to change.
I would keep yout blackberry plan as is and do not call to cancell anything. its not worth it. The good news is your blackberry data services are fully compatible with non-blackberry devices such as iphone. You can easily pop your sim card into an iphone and do not have to pay a DECF Data Early Cancellation Fee. You can use your existing data plan on a iphone without a hitch. You probaly dont want to upgrade just yet though, cus your still locked into your term so you may not be eligible to upgrade devices just yet id hold off and wait.