Harry didn't pay much attention to the testimony after the beginning, thus the timeline doesn't have to change if he goes back to make sure it contains some new False Memoried tidbits, if he can get someone to do the charm. But I don't think there's been much indication that he can override his Time Turner limitations by himself and there may be little time left to try and get someone to do it for him before Hermione is hauled off.
Edit: Silly me, he could just decide what to make her say later and do a quick check from McGonagall if they were included (thus checking if he will manage to go back to do the deed) and go from there. But it'd be difficult to insert subtle enough bits to make a difference only when lampshaded by Harry afterward (since many others presumably listened to the whole testimony already without noticing). Not impossible though. [Re-edited for semi-clarity...]
He could alter Hermione's testimony in a way that's contradicted by new evidence that hasn't yet been presented.
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: