I would really appreciate some advice

I received a PhD offer mid-December. Prior to this I had come up with a research topic, met with an academic in my home department (where I am currently studying), and she agreed to supervise my project. However, although she said she would supervise the research, the meeting did not go great. It made me realise that I really had nothing original to say in this particular area. There was also a distinct lack of chemistry, we didn't really get on.

But because I was fascinated by this topic, call it A, and know the literature, I just decided to go with it. Because after all, she had agreed to supervise me, and the course application deadline was looming and I didn't really have time to come up with a new topic idea. So, I wrote a research proposal and mentioned on my application that I had the okay from a department member, and got an offer.

The problem? I've changed what topic I want to research. I'm extremely anxious about this because the funding deadline is fast approaching, and I don't want to undermine my application by appearing like I just want to do a PhD on any old topic and don't really care what I research. I am still fascinated by A. But I am unsure what I can contribute, because it is a very abstract area. I find it interesting to read about, but just feel disheartened at the thought of having to write in the area. Meanwhile, Topic B, (different enough topic that I would need a different supervisor) has really captivated me. I've been thinking about B for a while, but have only now had the time to formulate a research proposal.

So I am stuck in an awkward situation, made more awkward by the fact that the supervisor is in my current department. Should I find a new supervisor to supervise B? I have 1 week until the funding deadline.

(sorry for mistakes, English is not my native language)