First it denied shooting down the Ukranian plane. Now it sez it saw it as a hostile plane and shot it down. Why change horses in midstream?I

Because they are caught.

This is how the world works.

Sometimes it is done on a grand scale, such as the decades-long campaign by the tobacco industry to deny nicotine is addictive.

Sometimes it is done on a personal level, such as yesterday at my place of employment where one of my co-workers made a mistake that could have (but most likely did not) cause damage to a customer's property. Our management decided to correct the mistake and proceeded as if it never happened.

When the USS Vincennes shot down Iran Air Flight 655, the US government first blamed Iran, then said it was an accident, then paid out a settlement to the victims (without admitting culpability), and found in its own investigations that the crew of the Vincennes had screwed up (e.g. they misread an altitude indicator which showed the plane was climbing, NOT diving to attack the ship).

Of course, I expect a huge majority of Americans have no recollection of the Iran Air Flight 655 incident and will claim Iran shooting down the Ukrainian airliner is evidence of them being soulless evil cowards. That is also how the world works.

The Iranian government is not generally stupid, despite its original foolishness in trying to pass the disaster off as something going wrong with the aeroplane.

It would have realised no-one believed its original denials but were prepared to accept it was a terrible mistake, especially once countries not so involved in the Iran / USA enmity started speaking out.

Really, the Iranian leaders had no choice, and once they came clean, it did not take long for many of their own citizens to show their own anger at the massacre. After all, many of the victims were Iranians.