The return will need to be amended to report the correct EIN on Schedule K-1.
From the IRS perspective, a different taxpayer received the incorrect K-1. The name and EIN won't match their records, so it needs to be corrected.
The investor should receive the corrected K-1 for their tax records. As long as the income & deductions on the K-1 didn't change, the investor doesn't need to file an amended return. If the IRS has questions, the corrected K-1 will explain the discrepancy.
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