GreenCardMA,
I did my 2nd FP on schedule. I don't think there is any advantage to do it early even if you are allowed to do so in some cases. USCIS will not even look at your case probably before the scheduled FP date. Look at current trend, it's my observation that most cases will be adjuncted 2 or 3 months after the SCHEDULED FP. Just as the 1st FP, the only "advantage" for early FP is, it will expire sooner based on the current processing speed.