Changing field for PhD - How much do I need to know before I start talking to advisers/applying?
First of all, do you have enough time to change your topic? If so, I would strongly recommend to talk to your supervisor first. If he/she knows you and your abilities, he/she can guide you on choosing a topic; you can finish up and get a PhD. Second of all, with a good help of your supervisor and your commitment you can finish a PhD.

Note on "Scary Parts": Nothing is scary!. I myself had no idea about a subset of computing semantics, and it felt like jumping from an airplane when I started to learn about it. However, at the end of the learning period, I realized why I was so afraid? Don't be scared, it will take your focus away, on doing your job. Your new slogan should be: Keep calm and learn!