Since you are already in contract and part of that contract pays your phone subsidy over the term of contract, it will still continue to be paid for in that contract.
Device swap is what you are going to do. Your current phone goes to sleep and your bought phone becomes active.
The link is for pre-paid, but is generally what happens with postpaid.
If you have a online Telus mobility account, you can use it to change phones.
Make sure the phone is compatible with the Telus network(no CDMA).
Is the person you will sell the device too another TELUS customer? If not, you might want to have the device unlocked for $35 before. To do this you can either call customer service or reach out to TELUSsupport via facebook or through a Private Facebook message on TELUS page.
I would have no clue but TELUS cannot guaranty the unlocks they do.
@ET: Do you mean Telus cannot guarantee unlocks done by third party unlockers? And that if a customer gets an unlock from such a third party, it is not certain the unlock will be successful and the phone work on the telus network?