Answer
I have swapped phones from Rogers, Telus and Bell countless times just as long as they are carrier unlocked. Although certain phones, despite having no reasonable reason, will not work. So remember to call and check first.
Not usually. You can keep your number now, but there are often compatibility issues with phones from different carriers. Even if your phone does seem to work, to reduce customer service headaches, the new carrier will probably require you to get a phone tuned for their system.
yes as long as you unlock it first