This is where hybrids come into play. You can't install the OS to your PC and just slap the usb cord in and expect it to work. There are major file differences for one being the .sfi (radio) file. What you can do though is what is called a full java swap. However there are certain Files that you need to delete from the 9700 OS for it to work. So to answer your question it can and will be done soon Posted via mobile device
Actually, this has more to do with the RAM, the OS6 builds for 9780 expect it to have 512MB of available RAM, the flash size on the 9780 and 9700 are actually the same (256 ROM). I would assume that some of the hardware addressing is a little different as well, considering that a few things have changed, (such as the 9780 having the 5MP camera instead of the 3.2 one found on the 9700). When the OS sees that there is a hardware mismatch of any kind, it's likely that the OS is going to halt, in order to try and protect the device from any permanent damage.
There are quite a few more differences on the board itself compared to the 9700. The power requirements on the 9700 are actually higher than those of the 9780. Be glad it stopped you, you could have seriously hurt your old device (although I doubt it would in all reality).