Typically employers state this type of change in their open enrollment materials, and then treat that as a Summary of Material Modification. Technically they should also amend the combined Plan Document/SPD to reflect the change in carriers. The amendment could be a simple single state that explains the change in carrier (i.e. MultiState will now be replaced National). If the open enrollment materials were used to announce the change, the participants are not required to receive the updated plan/SPD.