Rogers only told us about the iPhone 4 upgrade policy right before it came out, so I'd say you should wait about 11 months for a for sure answer to that question.
But Rogers' history, from what I remember:
- 3G Launch: people who upgraded 2 months ago or longer were allowed to upgrade at the 3 year new customer price (the "low price")
- 3GS Launch: 3G upgraders were allowed to upgrade, but for something like $399/499. Don't remember for sure cause I didn't do it.
- 4 Launch: People who got the 3GS and the 3G both at their launch were allowed to upgrade for the low price. 3G only upgraders had to have done it before Dec 31 2008 to get the low price. 3GS only upgraders at launch got it for 399/499.
By "launch" I mean a few within a couple months of the phone coming out (6.5 months for 3G, 2 for 3GS)
So if this is your first iPhone from Rogers, I'd say you'll have to pay $399 for the base model iPhone 5 next year if Apple keeps the prices the same. If you have the 3GS, and get the 4, you should get the low price at launch of iPhone 5. Obviously this is just my speculation based on past Rogers pricing, I have no inside knowledge (duh).
This is where some doofus comes along and says "But we don't even know if there will be an iPhone 5 next year!", but still, someone feel free to correct me cause I know I'm wrong on some of it.