I would suggest filing a final return under the grantor trust EIN from 1/1 to date of death recording the income earned during that time on the final 1040. Then set up the new EIN for the simple trust and fund it with the assets from grantor trust. You will need to re-register the assets under the new trust name and EIN at the brokerage houses. It might be cleaner if you just set up a new account under the simple trust name and EIN then transfer in the assets versus just changing the name on the current account.. Prepare the 2020 1041 for the new trust with income from date of death to 12/31 flowing out to beneficiary on the 1041 K1. You could check with the brokerage firms if they could split the 1099s based on the date of death.
