I have requested a new N number for my airplane a few months ago. Thinking the change would come through I had my airplane painted with the new number. What will happen if I fly it without the number being authorized?
As a controller, if it's a correctly constructed N-number, I'd know nothing. and wouldn't do anything. If you file a flight plan, however, the N-number you use will go into the FAA's database. Keep in mind that if you ask a controller for flight following, a Mode C check, etc., there will be a record, even if it's only "on the tape".
I have no idea whether they would flag an unregistered number, but I can give you some history on an aircraft that flew with an INVALID N-number.
In 2006 (or so, why is this stuff so long ago?), I was working the high altitude sector in the LBF (North Platte, NE) area. I received a handoff on an aircraft with a call sign similar to, but not exactly "N1EP2D", which is an illegal construct (see https://www.faa.gov/licenses_certificates/aircraft_certification/aircraft_registry/forming_nnumber/)
I alerted my supervisor, who directed me to question the pilot. As it turned out, the pilot had filed this deliberately, and on a whim.
The FAA does not do "whimsical".
I instructed the pilot to submit the correct N-number, which he did, and I amended the flight plan. I also instructed the pilot to NOT use the invalid number, and to call our Operations Manager for further instructions.
I heard it was a six-month suspension, although I had no official follow-up.
You cannot fly. Since you changed the N-Number, you no longer have a valid airworthiness certificate (since they are tied to N-Numbers). This isn't just a wrong number on the side issue, you don't have a valid registration or certificate. You have 10 days after getting the registration number on your aircraft to file the appropriate form and receive an updated airworthiness certificate.
As far as what can happen... that depends. The FAA has pretty wide authority to issue fines, revoke certificates, or even impound aircraft. There is no set "you do X, then you get a fine of Y" like for traffic violations.