If the business filed Form 2553 Election by a Small Business Corporation, the business would have to revoke (or terminate) this election for it to no longer be effective (once the IRS approves the revocation). A business should continue filing as an S Corporation until a termination of the election is made and approved by the IRS.
Per IRS Instructions for Form 1120-S U.S. Income Tax Return for an S Corporation, on page 2:
Termination of Election
Once the election is made, it stays in effect until it is terminated. If the election is terminated, the corporation (or a successor corporation) can make another election on Form 2553 only with IRS consent for any tax year before the fifth tax year after the first tax year in which the termination took effect. See Regulations section 1.1362-5 for details.
An election terminates automatically in any of the following cases.
Per IRS Instructions for Form 1120-S U.S. Income Tax Return for an S Corporation, on page 3 (third paragraph of When to File):
When to File
If the S corporation election was terminated during the tax year and the corporation reverts to a C corporation, file Form 1120-S for the S corporation's short year by the due date (including extensions) of the C corporation's short year return.
Note. You may file a fiscal year or short year return using the TaxAct® 1120-S program. The IRS allows e-filing of short-year returns if it is the corporation's first year or final year; otherwise, a paper return must be filed instead.
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