When it happened to me, I stopped by their office the next time I had to fly. I didn't know there was an option to do it online. One benefit of doing it in person is that they will put the sticker on your new passport, which is what will allow you to skip to the front of the line if you ever are in a situation where you're at a GE port of entry but the machines are down. It will NOT let you skip the line at non-GE ports, such as the Port Canaveral cruise port.
Although of course GE benefits aren't activated until after your interview and final approval, I've heard rumors that your KTN will work with PreCheck as soon as you receive your notice of provisional approval for GE. This presumably is because if you don't qualify for PreCheck, you're definitely not going to get GE, so if you get provisional approval for GE, you've already met all of the background requirements for PreCheck. But, that would potentially put you in a situation where you haven't been fingerprinted yet, so that would seem to be a potential issue. I guess that once you know your KTN, it wouldn't hurt to put in your airline profiles and see what happens.