U have to tell them. There is

They will be angerier if they found themselves doing something illegal because you didn't inform them.

You can just tell them that you were not aware that you have to let them know. And you don't need to tell them you filed 485 anyway, if you intend to maintain your F1. But you SHOULD have told them when you wanted to extend your I-20.

Have you been working after your I-20 extension without EAD? That is, did you get any TA/RA payment since the extension?