To the best of my knowledge, I`ve never heard of alternative charges being introduced mid trial, before the trial, yes - normally a lesser offence, as part of a plea deal, to avoid dragging witnesses to court or putting fragile witnesses through the ordeal of giving evidence.
Murder is quite a simple offence really, but can be difficult to prove, as there has to be either an admission, a clear intent to kill or the actions of the defendant where so reckless, that there could only have been one possible outcome - death.
For instance, if someone hit another person square in the head with something like an axe and the person died, it would be difficult to argue that other than in self defence ( the other person had an axe etc ) that hitting someone square in the head with an axe, could only lead to one outcome, even if the person swinging the axe says that they didn`t mean to kill the other person.
If the same person, hit the other person over the head with a plank of wood and the person died, the defendant would have a strong case to argue that he didn`t mean to kill the other person, just hurt him, as he only hit the other person, with a plank of wood, which wouldn`t normally be enough to kill someone.
In the case of the policeman, he`s argued successfully, that even though he shot the lad through the window, which met the evidential test for intending to kill him, he was in fear of his life and therefore it was justified.