It will most likely be similar to the 3g --> 3gs...
People fail to appreciate just how much of an improvement the 3GS was over the 3G. Sure they looked the same, but the technology jump was far greater than the 2G -> 3G upgrade.
I'd go so far as to say the improvements from the 3G to the 3GS were more significant than the 3GS to the 4 upgrade.
19 months after release, a 3GS still runs all current apps and the current iOS very well. At this same point in its lifecycle, the 3G was really struggling to perform.
We would be lucky to have such an upgrade with the iPhone 5 (or iPhone 4s) this summer