You are able to upgrade your device at any time during the contract, so long as you are in a good credit standing with Telus, by simply paying your device balance. Your device balance starts out at the discount you got on your device (ie. $500) when you first signed the contract and then goes down every month until it eventually reaches zero at the end of the contact. You can find your device balance through your telusmobility.com account or by going into a Telus store. Seeing as you have a high end smartphone and are only a year into the contract your device balance will likely be quite high. To offset this you can trade in the S3 which should still have a decent value. One thing to consider though is that you can probably get more money for the device by selling it privately on a site like Kijiji. Where you might get $150 from Telus for it you could get $350 on Kijiji. Or there's always the option of buying the S4 at full price, but that's going to set you back a few pennies.
