If he could change part of the testimony to something demonstrably false, that no one else in the room knew at the time, he could prove that her mind had been compromised. Actually changing the memory would be a problem, and it doesn’t seem like a likely solution to me, but it’s still possible.
But since the audience’s (extended) reaction includes voting to send Hermoine to Azkaban, how will changing her testimony help?
If he could change part of the testimony to something demonstrably false, that no one else in the room knew at the time, he could prove that her mind had been compromised. Actually changing the memory would be a problem, and it doesn’t seem like a likely solution to me, but it’s still possible.