The low-res screen concerns me - if the next gen is indeed higher res which it probably has to be then what happens to the apps? iPhone apps look ridiculous, will the older iPads be able to downscale iPad 3 apps or will they simply not run? If there ends up being multiple variations of everything that will only lead to awkwardness for developers and less support for these earlier models.
If the next iPad has a higher res screen, it will be exactly the same situation as when we went from the iPhone 3GS to iPhone 4. Everything will just work. The only thing developers will eventually need to do is provide higher resolution versions of any bitmap images in their apps, but no massive redesign will be necessary. The lower res iPads will still be able to display these apps just fine, the same way an iPhone 3GS today can still run apps designed for the iPhone 4.