S Corp Converted to an LLC

But you would have had to liquidate the S corp in order to transfer assets to an LLC.  You have two separate entities and therefore would have two separate tax ID numbers --------------------- with some potential tax liability upon liquidation of the S corp.

One doesn't "convert" a S-Corp to an LLC.

Your client has a dissolution, with the assets distributed (at fair market value) to the shareholder(s) AND any resultant gains reported/taxes paid.

Then the assets (some, or all) are contributed to the LLC, as well as (possibly) any associated liabilities.

Off the top of my head.. but I'm thinking a new FEIN is required (unless it's a SMLLC without payroll).

To revoke a Subchapter S election/small business election that was made on Form 2553, submit a statement of revocation to the service center where you file your annual return.

The statement should state:

• The corporation revokes the election made under Section 1362(a)
• Name of the shareholder(s),
• Address of the shareholder(s),
• Taxpayer identification number of the shareholder(s),
• The number of shares of stock owned by the shareholder(s),
• The date (or dates) on which the stock was acquired
• The date on which the shareholder's taxable year ends
• The name of the S corporation
• The S corporation's EIN
• The election to which the shareholder(s) revokes
• The statement must be signed by the shareholder(s) under penalties of perjury
• Signature and consent of shareholder(s) who collectively own more than 50% of the number of issued and outstanding stock of the corporation, (whether voting or non-voting)
• Indication of the effective date of the revocation (or prospective date)
• Signature of person authorized to sign return

Due Date of Revocation:

• If revoking effective the first day of the tax year, the revocation is due by the 16th day of the third month of the tax year,
• If revoking effective any day other than the first day of the tax year, the revocation must be received by IRS by the requested effective date.
  • For example, the S corporation is on a December 31 tax year ending and requests a revocation effective Jan. 1st, the revocation is due March 15th.
  • The S corporation is on a December 31 tax year ending and requests a revocation effective Feb. 14th, the revocation is due Feb. 14th. 

https://www.irs.gov/forms-pubs/revoking-a-subchapter-s-election