In the middle of Act Two, the 5th Juror changes his vote to not guilty. Is this purely a result of the 8th Juror's argument, or are there other factors involved?

No, his hesitation isn't related to the 8th Juror's argument. When the 2nd Juror brings up the coroner’s testimony that the father had been stabbed in the chest “down and in” and also that his father was six or seven inches taller than he. While reenacting the mechanics of this stabbing, 5th Juror gets up and begins to examine the switchblade. He explains that it would make no sense for someone to stab someone like that with a switchblade because it would require the attacker to lose precious time. Because the defendant was highly competent with a switchblade, it becomes questionable whether or not he would have made that chest wound.