That's great that you have another opportunity to study something which interests you more. You can make a new start! This is a good thing - you just need to extricate yourself from your current situation. Yes, it could be tricky telling them that you're already studying towards a PhD, but Teek's suggestions are good. The other thing you need to to consider is that academia is a small world, and if you remain in the same area, will your past and potential supervisors and colleagues know each other? Are they likely to run into each other at conferences etc? If they are, and they start discussing you, this could get really awkward. I think you need to tell all involved about your plans - don't let your potential supervisors find out you're already studying/have started a PhD through the grapevine.
You need to have some hard conversations, but just think how much better you'll feel when this is all done, and you're settled into your new project!
I've had a few jobs where I've quit in the first couple of weeks to move onto something more suitable, and had to have difficult discussions, but did it, and never looked back. You can do it too. Do what will make you happy.