We're not rewriting history. Allowing a transgendered person to choose a new name doesn't change the basic facts about what happened. It's just an act of compassion towards someone who would otherwise be suffering from feelings neither you nor I personally experience, but are well documented by scientists.
> Shouldn't it be "Bradley Manning (now called Chelsea)"?
You could say "Chelsea Manning (at the time, 'Bradley')" if you really wanted to refresh people's memories, so long as you consistently refer to her as Chelsea through the rest of the comment/essay/etc.
> I prefer objective to polite.
This is a false dichotomy. You can be both.
Someone changing their name and identifying as the opposite sex after being arrested (the next day I think?) in a major political scandal makes that pertinent to the history of the situation. Hiding that by pretending the person always was identified as they currently are hides facts about the situation -- are they pertinent, maybe not to the primary issue, but the contemporaneous nature suggests something of interest (in social history if nothing else).
Are you asking in your position as moderator?