Signed an offer, then they posted the role I actually wanted. Should I tell HR?

I signed an offer with a company a while back and I start in October. This week I noticed they've posted another opening that lines up much better with my skills and background, and honestly it's the role I'd have applied for if it had existed at the time.

Would it be reasonable to email the HR person I've been dealing with and mention that my profile might be a better fit for the new role? Or does that come across as ungrateful / flaky before I've even started?

I don't want to blow up the offer I already have. Just wondering if this is the kind of thing people bring up, and how you'd word it if so.