As far as I know, you can buy out your retail contract at any time and enter a new contract. You may not be able to upgrade to a new phone on your current contract, but you are not confined to your current contract, and your obligation should end upon payment of your outstanding balance fee. Corporate (employee purchase plans) may be exempt from this option, if you have one of those deals.
Just to note: After reading around quite a lot I was under the impression that Telus waived the remaining 100.00 or less of a device balance when a customer upgraded with a new contract. Not necessarily so. I upgraded yesterday from the Samsung S4 to the S7 and had the option of paying the remaining S4 device balance of 75.00, or adding it to my next bill. My daughter had the same options for her remaining 48.00. So it goes for some.
We had awesome customer service though. Considering we took up 90 minutes of the service rep's time and the place was packed with customers, he was fab, fun, and so helpful. I guess he was worth 75.00. 🙂