Editing my initial answer since I misread the question . You are asking if you can amend the plan by 12/31/2025 for a 2026 change from SHM to SHNE?
Technically, you can make the change at any point before the plan year starts. A question that could be asked is whether participants will have a reasonable time to make changes to their elections after you amend, and if they are negatively impacted by the change if they do not have reasonable time to change their elections. I think you are fine since you are providing the SHNEC in place of the SHM and participants will not miss out on any of the employer contributions even if they do not have enough time to change their elections for the first payroll. It would be different if you went from 3% SHNEC to 4% match and a participant did not have time to change their election to receive the full 4% match.