Sure they can just tell you to come back on your appointment date, you won't lose anything, you can only win. This is mostly for the people that live close by to their ASC office and can stop by and ask if they can take them earlier. If not, they will just go back on their appointment date. I would post the link for you to the other forums where people are already getting their EAD sent to them because they went in earlier for their Biometrics, but I don't think posting the link would be right.
I will probably be calling in and asking them if my sister and I can go in at the same date because I sent her forms 2 days later and she will most likely get a different appointment date because it is too far to drive down there twice.