There is no official guidance on the matter yet, but my understanding is:
They can amend the plan now, or any time up through November 30, 2020, to be a 3% SHNEC for 2020, effective retroactive to 1/1/2020
They can amend the plan any time up through December 31, 2021 to be a 4% SHNEC for 2020
Under either of the above, the plan will be exempt from the ADP test for 2020
The plan will be subject to the ACP test on the match, since the ACP safe harbor still requires a notice before the beginning of the plan year