The question is: what will be the models still sold after next autumn refresh.
Another big question: has Apple interest to still sell devices not able to run every services (Siri, Map, etc.) of iOS6?
If the screen changes such that the resolution is different to actual retina iPhones, you can consider the 3GS will disappear, to avoid fragmentation of iPhones in use,
and adding the fact the 3GS hardware will probably not be able to run on the 2013 iOS iteration, Apple will probably not sell a device becoming obsolete the next year.
Now, is the iPhone 4 so different to the 3GS internally, in term of performances (not considering the screen)?
Has Apple interest in stilll maintaining production of iPhone 4 and 4S, while a new iPhone with different process is also in production?
While Apple needs low cost phones to sell as the 3GS is now,
i still can imagine they maybe will sell beside the new iPhone only a 4S (retina 960x640, antennas ok, 3G only, full iOS6) with only 8Go, and make it as cheap as possible.