Look behind Spoiler below:
Spoiler:
Here is the hybrid SD kit as a zip file: [Only registered and activated users can see links. ]. It has Ldr.bin (boot.bin) from 3xx and 14.bin (fw.all_bin) from 6xx with HWID changed to 0566 and SWVer to 520. Extract the zip file directly to a small clean SD card. Don't use a large card or the device may not read it, even 256MB should suffice. I've included in the txt file the command to dump the nonvol memory region (rgn41) however it may be protected in older devices like 3xx & 6xx. With device fully off, insert the SD and then turn on. Hopefully it will read the card and even if the command to dump rgn41 fails, it should successfully flash rgn14 with the 6xx fw_all data from 14.bin. When it finishes, fail or succeed, it should reboot and if successful will show the normal logo screen and then operate like a 6xx even though it still has HWID 0566. If so, check if it can access preboot and be seen by Updater.exe then report back. NOTE: Probably goes without saying but this is all at your risk. If this works and it does have preboot access again we'll then try to flash the full 6xx HWID 0596 as an RGN to fully restore it.

Good luck, this should work. I don't think it's likely to damage it further but if it does it's toast most likely.