EIN Mixup: Used New LLC EIN on Previous Year's Sole Proprietor Return

[mogwai]

Background

• In 2023, I operated as a sole proprietor
• As of January 2024, I transitioned to a single-member LLC taxed as an S-Corp
• I received a new EIN for the LLC

Issue

When filing my 2023 tax return, I mistakenly used my new LLC's EIN instead of my sole proprietor EIN. The return has already been accepted and processed by the IRS.

Questions

1. What are the potential implications of this EIN mixup?
2. Should I file an amended return or can I take other corrective action such as sending a formal letter?

Any insights from those who have dealt with similar situations would be greatly appreciated. Thank you for your expertise!

[MaryK4]

So, a single-member LLC taxed as an S-Corp is not correct.  The LLC can be a SMLLC which reports on the Schedule C OR it can elect to be treated as a Corporation that elects S Corp status.

See Do you need a new EIN?  You may be able to correct the Schedule C because you did not intend to file with the EIN until 2024-but I am not 100% sure how- perhaps you could send an amended return with the explanation on the 1040X.  Technically you would need a new EIN because you used it for your SMLLC in 2023 then changed to SCorp in 2024, but once again because you did not intend to use the EIN under the SMLLC, it seems unnecessary.