Canon has been offering discounts and cashback on many items, every few months a new set of discounnts is offered, so a cashback does not signal a repolacement model.
The low end Rebels are usually upgraded approximately once a year. They usually have minor changes, mostly issued so that new buyers who want the latest and greatest will go for it. Its unlikely, however, that a new model will have anything that a existing 600D owner would want to upgrade for, you usually need to wait for 3 years or so to see any significant changes that have crept in over time.