Late S-Corp election, but Corp return already filed...

For 2018 TY client THOUGHT a 2553 S-Corp election was filed for his C-Corp but never got an acceptance form IRS. So he filed a 1120 for 2018 TY before the deadline while waiting. Turns out IRS never received the 2553. So I believe we can still retroactively apply for S-Corp status, however - How are the 2018 taxes handled? If/when 2553 is accepted, do we file an 1120S for 2018 and "overwrite" the 1120 filed return? What is the process for "correcting" the 1120 return? Thank you!

How are the shareholders going to satisfy section 5.02 of Rev. Proc. 2013-30 when the corporation filed its return as a C corporation?

.02 Supplemental materials. The completed Election Form must include statements from all shareholders during the period between the date the S corporation election was to have become effective and the date the completed Election Form is filed that they have reported their income on all affected returns consistent with the S corporation election for the year the election should have been filed and for all subsequent years. Such statements must comply with the requirement in Section 4.03(3) of this revenue procedure.

They did so in error/fear of not filing anything before the due date.

That doesn't answer my question, so maybe I need to ask it another way. Did the shareholders file their individual returns as if the S corporation election had been approved?

Sorry, misunderstood. Single owner for the business and he did NOT file his 2018 personal returns yet. I guess another way to ask is can a 1120 be amended with an 1120S if a retroactive 2553 is accepted by the IRS?

The 2018 1120 was filed with all zeros.

Go through the flowchart at the end of Rev. Proc. 2013-30 for a late S corporation election:
https://www.irs.gov/pub/irs-drop/rp-13-30.pdf

If you can get the shareholder to file his 2018 return as if the S corporation election had been approved, and then file a 2018 Form 1120S with Form 2553 as described in section 4.03(2)(b) of the Rev. Proc., then I believe that this may work.

Thanks for your insight. I saw that too, but wasn't sure if it was applicable if the taxpayer ALREADY filed an 1120.