You can't upgrade from a 32-bit os to a 64-bit os. It requires a clean reinstall. If you do the windows 10 upgrade it will be a 32-bit OS. Your windows 7 key should work for both 32-bit and 64-bit versions, you'll just need to find media. After you get 64-bit win 7 media backup your data and do a clean reinstall of win 7 64-bit, then you'll be able to upgrade to 64-bit win 10.