Changing the main topic of research
There's only one thing to do here, and it is having a talk with your thesis director. He's most likely the best qualified to :


answer your worries with A being not valuable enough and if it's worth pursuing
assess the scope of the grant and the leeway you have with this


Doing a thesis on a subject is a commitment. Don't give up because you 'feel' it's going no where, prove it's not going there first. Also, finding similar results in different initial conditions, while not exciting regarding finding the next innovative thing, is not necessarily worthless. It's good to have results doubled checked/confirmed, found by other methods, or  if another result is expected, the work also serves as excluding things that don't give this expected result. Which is a result by itself.