I think the extension of the 1040-ES is included in Notice 20-23 :Expanded Federal tax filing and payment relief on account of Coronavirus Disease 2019 (COVID-19) emergency. The Treasury Department and IRS are providing additional relief to taxpayers, postponing until July 15, 2020, a variety of tax form filings and payment obligations that are due between April 1, 2020 and July 15, 2020. Associated interest, additions to tax, and penalties for late filing or late payment will be suspended until July 15, 2020. Additional time to perform certain time-sensitive actions during this period is also provided. This notice also postpones due dates with respect to certain government acts and postpones the application date to participate in the Annual Filing Season Program. This notice expands upon the relief provided in Notice 2020-18, issued March 20, 2020, and Notice 2020-20, issued March 27, 2020.

Included in that variety is 1040-ES. I copied the above from the guidewire sent 4/9

I am afraid changing the letter is fine, if the client is actually mailing the estimates, but we probably need to wait for software to update if we are doing direct debit.