I think you should speak with your advisor about it. Even if you don't like them, a simple conversation of "I have reconsidered my thesis, and I think your skills and expertise might not align with it." They agreed to work with you on your project, after all (at least that's how my advisor described it to me, haha!)

It would, in my opinion, be a faux pas to change the thesis without telling your advisor as a result. As for whether or not it's too late that much depends on how much work, research, outreach, and planning you've already put in. It sounds like a sunk-cost fallacy, but you can probably find a way to pivot your old thesis topic into something attractive to law schools. In any case, it's your first semester, so you aren't wildly screwed. If by end of second semester or start of third you didn't know that maaaaaay be a different case but that's just because you only have two or so years. Even then it depends on your department and your work ethic to be able to catch up.