There was no further investigation before the drowning cause of death was changed. It is very plain in the article.
I know what the article says. I don't know what makes the author so sure she knows that no further investigation was done, or simply that the medical examiner didn't simply reconsider all the evidence. She says herself that she hasn't been able to get any information about why the cause of death was different.
I also note that the original police report doesn't actually say the preliminary cause of death finding was based on the autopsy. The preliminary cause of death was probably drowning from the moment they pulled her from the ocean. It's also possible, of course, that Detective Winfrey simply made a mistake. I just think the author is extrapolating way too much from the limited facts at her disposal.