Knight has not been a model prisoner but, it seems, only because he upset the Government by challenging them over prison conditions etc., so that should not be held against him.

"Is it right to change a prisoners sentence"
Of course it is not right. A sentence is a sentence and should be finite.
However having said that there is a quite legal existing way to change a prisoners sentence.
The sentencing judge can call the prisoner back to court and re-sentence him.
He could change the sentence to a period at the Queen's Pleasure which might see the prisoner die in prison.
Of course if you are going to keep someone in prison indefinitely then you may as well bring back the death penalty and save a lot of ongoing expense.
As far as Mr. Knight is concerned he will kill again because his hatred of police is no more ingrained though cleverly hidden but he will kill again.