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

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!