You can change a patent before a notice of allowance has been sent, so the answer is yes, but you may not receive the same filing date. Why? Because that's how the system maintains fairness to inventors.
Let's say I invent a chair. It has arms and a back, but no wheels. I file a patent for a chair with arms and a back. A few months later, I decide the chair may be improved if it has wheels as well. I can go back and change my patent to a chair with wheels and add a claim for the wheels. But I will not receive a filing date for the wheels which is the same as the rest of the chair. On the day I filed the chair application, I hadn't invented the wheels yet. So I wouldn't deserve a filing date a chair with wheels on that day. Some other inventor may have thought of the wheels addition before me. If he did that, he could file for the wheels on the chair and he would be entitled to his filing date before the filing date of my wheels, and that would be fair because he invented that improvement to the device.
There are several ways to go about making this change. I can file a continuation-in-part application, a continuation application, or a new non-provisional. Some of those options will delay the filing date of my entire chair, and some will not. But, in no event will the patent office grant me a filing date for wheels on a chair before I invented and filed for the wheels.
We just sent you an email. Please click the link in the email to confirm your subscription!