My question is: was your reading the first reading or the second reading? If you were only given a first reading, then your unit can pull back the CG Art 15, and read you a FG Art 15. If you were already given your second reading for the CG Art 15, your company commander already signed off on your CG Art 15 punishment, then by reg, you have received your punishment and you serve out whatever sentence you received for your CG Art 15.
If it was only the first reading, the answer is yes. The command can change it to make it a field grade. If you have already been there, you would need to go BACK to TDS (Trial Defense Services), because the possible punishment at field grade is more severe and you need to know this as well. You may well decide that you now do not want the matter to proceed by Article 15 and that you want a court martial. This is a matter that you really need to talk to a TDS attorney about. IMPORTANT,dDo not exercise your right to go to court martial rather than 15 without first at least talking to a TDS attorney about your case. TDS is free, you are entitled and indeed required to at least have the chance to consult with them.