The answer to the original question is that upon the death of the grantor, the trust becomes irrevocable and any income received by the trust after the date of death needs to be reported under the EIN of the trust. No income after the date of death resulting from investments held in the trust is reportable under the decedent's SSN.
If gross income to the trust is more than $600 in any one of the trust's tax years, the trust must file Form 1041.
If mistakenly reported under the grantor's SSN, the payer should be asked to correct the reporting to show the payee as the trust under the trust's EIN. If the payer will not make the correction, the nominee process can be used to move the income to the trust.
Whether or not the income to the trust can be distributed to the trust beneficiary and the tax obligation passed through to the beneficiary on Schedule K-1 (Form 1041) is a separate issue dependent on the terms of the trust.
