Not the CARES Act, the Secure Act.
A 401(k) Plan that is NOT a safe harbor can be amended to a 3% Non-elective Safe Harbor in the first 11 months of the plan year.
A 401(k) Plan that is NOT a safe harbor can be amended to a 4% Non-elective Safe Harbor in the 12th month of the plan year.
For a regular 401(k) Plan, you can not add a Safe Harbor match mid-year.
For a Safe Harbor 401(k), you can not change the type of Safe Harbor; match to Non-elective or Non-elective to match mid year.