I had the same experience a year ago - basically, 2 months into my PhD, like you, I decided that my proposed topic wasn't for me (it just didnt 'grab' me).
So, I wrote a new proposal and emailed it to my supervisors and asked them for feedback. I made sure my new proposal was better (i.e. tighter, more developed) than my original one, and made sure I came across as really enthusiastic about the new idea, to the point where i didnt think they could legitimately refuse (which they didnt). It was a very different topic, although within the same discipline (psychology). It might be harder if you want different supervisors as well though.