If you changed your monthly plan & receive a promotion on that plan, your contract will most likely be extend,depending on the promotion, another 2 years or 3 years starting the date you switch you plan. If you simply changed your monthly plan without receiving any new promotion, cuz you already receive one when you signed up the first time, Rogers can't automatically extend your service contract.
The contract says you've to stay with Rogers wireless within the contract period; but never mention that you've to stay on the same plan forever. You'll be converting to a monthly billing after the contract service period ends.
Ask carefully before switching your plan, I got mine changed & its was better than the plan I receive when I first sign up, so I don't mind extending the contract. Again, ask before you do anything or dont blame Rogers not telling you the facts.