I don't expect answers too quickly, but I have kept the Ash-Greninja on my SM Demo and I want to know if it is possible to transfer/trade Ash-Greninja to Ultra Sun/Ultra Moon.
Pokemon Ultra Sun and Ultra Moon will most likely be compatible with Pokemon Bank, so yes!
No, no official way to transfer has been yet confirmed. But you can easily get Ash-Greninja on Ultra Sun/Moon by Island Scan, as far as I know, somewhere on Poni Islands I don't remember. Best option- Pokemon Bank!
You can not transfer it from the Sun and Moon Demo. So your only option is Pokemon bank.