Law enforcement sources tell TMZ the reason for the change is that there was bruising on Wood's body that is not NECESSARILY consistent with an accident ... but it doesn't prove foul play.
Here's the real reason for the change. After Wood's died in 1981, then L.A. County Coroner Thomas Noguchi jumped to a conclusion -- something he was prone to do -- that Wood's death was an accident.
Noguchi did not follow protocol by accounting for ALL possibilities for the bruises -- possibilities which could include foul play, but could just as easily come down to Wood's hitting the side of the boat or the dinghy as she fell in the water.
We're told protocol all along should have been to call Wood's death "undetermined."
Our sources say the investigation is still officially "active" but in fact it's been at a dead end and detectives have unearthed no new evidence to lead them to a conclusion there was foul play. We're told there is "certainly no evidence" leading detectives to a suspect.