I think it’s hit or miss, especially on launch day. Last year I had an 8:30 am appointment and I showed up at about 8:00 figuring the worst that would happen was that I’d have to wait 30 minutes. Initially they told me that I would need to wait until my scheduled time, but then at about 8:20 there was a gap in the line for people with appointments and they took me then.
If your appointment is at a crazy busy Apple store then you’re probably less likely to be taken early, but if it’s a quieter one and particularly not first thing in the morning then you might have a chance.
If it’s not too far to go it might be worth going at lunch and seeing if you can get it early.