If they actually have learned, and they're still keeping quiet about it, one can only speculate that WP8 is going to represent such overhaul of the hardware specs that it will be too difficult to support the existing devices. Maybe there's some truth to the rumor about WP8 sharing the same kernel as Windows 8.
Referring to Hardware Specifications for Windows Phone it seems like the hardware is already ahead of Windows "modularized". Additional hints ... Microsoft wanted no-native on the WP7. I'd imagine so they could bring the WP7 market and the new WinRT market together at some point in the future. Remember the goal is Windows Everywhere. Given that hardware today (even on the smallest devices) can run general purpose OSes it seems the goal of applying the Windows OS across all devices is achievable. Combine what we know about Ballmer's statement of Windows everywhere and what we have heard about Windows being more modular and the plan seems pretty clear.
Since there are no Windows Phone 8 hardware specs publicly available yet, we can't verify how far along the merge paths Windows Phone 8 sits. Is it the final merge point before Windows everywhere or is there some more to go? Too soon to tell. In addition the merge paths will more than likely have to be amended because there are other competitive market forces in play and they are some big boys too.
Ultimately there may be no transition from WP7 to WP8 not because of the OS changes but because of the wireless carriers insistence on controlling the software on the devices. Maybe leaking to the market there is a new Windows 8 for phone in the works is designed to put pressure on the wireless carriers to allow OS vendors access to their OS customers and by extension allow software vendors access to their customers.