I agree it looks very much grafted from what I can tell. If you care to have a great tree in the future (or at least a good one) you will need to address the graft as the union tends to get worse with time. You'll do this by air-layering the tree but that's another story for another time. However, if you do not care if you ever have a good, showable bonsai, it is perfectly fine to leave it as is and just enjoy growing it in a container. Japanese maples are beautiful refined trees that bring something different with every season. If you are relatively new to bonsai, I would not worry at the moment about the graft and concentrate in learning to care for it and ensure that it thrives under your care.
Like others have mentioned, wait until the spring to repot.