Q: If my ratio or my download-amount crosses one of the threshold boundaries, will my account become limited to seed-only in the middle of a download?

A: Yes, it will, and you may in fact find yourself unable to seed back because you do not have the 100% completed torrent. This is why it is important that you anticipate when you will reach the next limiter threshold and plan ahead to have your ratio above the required level when you cross it. Again, if you maintain your ratio at 1.0 at all times, you will not have to worry.