A tab is like a floating loan Amanda. You don't "buy it out" per se. Let's say you have a $150 Tab Small which you still owe $80 on. Let's suppose you want to remain on Tab Small to get your new phone. That would essentially give you $70 off the price of the new phone you're buying (because Koodo forfeiting you $70 would take your Tab Small back to -$150, just like a new Tab Small). You can stay within that bracket for as long as you want (what I mean is, you can take it back down to -$150 as many times as you wish).
Now, I'm not giving out examples with Tab Medium and Tab Large because there are other metrics at play, but that's essentially how it works.