By contract, are you referring to the tab? Koodo has only had contracts in the past for iphones and not the s3. If so yes you can sell your s3. BUT before you can use the tab again to upgrade you have to pay off your current negative tab. Yes you can keep the same sim if you are wanting a 4s. If you want a 5 or newer which I'm assuming you are, you would need to buy a nano sim for 10 on postpaid and 20 on prepaid. you are able to swap sims in selfserve. :)
Yes your tab is tied to your account not the phone. Keep in mind that to get a new phone from Koodo you will need to pay off your tab in full at the time of purchase of the new phone or on your next bill. If you already have the iPhone then no need to pay off your tab in full it will just carry on being paid like it has been. If I the iPhone takes a different sized SIM Card which chances are it does you will need to buy a new SIM Card which is $10, however if it's the same size SIM Card you can just pop it into the iPhone and away you go, as long as it's a Koodo iPhone or its unlocked.