Re-Paying sign-on bonus?

Last year I was hired into a job and received a sign-on bonus and the terms stated I would be asked to repay it if I left within 12 months. I left after 7 months, however, I heard no mention of the bonus until just this week, and it's been 6 months since I left the company. Is there some sort of statute of limitations for this? Some of the statements made in my offer letter (specifically, who I would be directly reporting to) were changed before I started and I was not notified, so I feel like they didn't uphold their part of the employment agreement (which is why I left). Would this have any effect on whether or not I have to repay it? What happens if I just ignore the letter?