You obviously have more concrete experience here than I do, but my instincts say there's a vast gap between "working on a side project you hope to eventually work on full-time" and "form an official registered company with the IRS".

If you go to the IRS / your state's department of commerce and say "hey, we JUST TODAY started a BRAND NEW COMPANY that happens to already have X million users and we want to incorporate", do you know if they'd snoop around and accuse you of having "started" it prior to talking to them?