Easy one first, there is nothing you need to notify IRS you are no longer Self-Employed.
OK now the estimated tax payments. You did work as self-employed from Jan-first part of March.
You can pay the first estimate and be done with it, or split it up and pay the first one over 4 quarters. Choice is yours, I like to keep things simple, so if I have the money pay it and be done for the year.
