for (int i = 0; i < ac.Count(); i++)
{
     if (ac.ElementAt(i).IsRelationship)
     {
           //Если изменена связь

          //Получаем какие поля у таблиц связывает эта связь
          //???
          //ac.ElementAt(i).OriginalValues[0] - наш первый элемент в связи
          //ac.ElementAt(i).OriginalValues[1] - наш второй элемент в связи

     }
}

        <Association Name="FK_LandRentPretender_Person">
          <End Role="Person" Type="EstateUOModel.Store.Person" Multiplicity="0..1" />
          <End Role="LandRentPretender" Type="EstateUOModel.Store.LandRentPretender" Multiplicity="*" />
          <ReferentialConstraint>
            <Principal Role="Person">
              <PropertyRef Name="OID" />
            </Principal>
            <Dependent Role="LandRentPretender">
              <PropertyRef Name="SubjectPerson" />
            </Dependent>
          </ReferentialConstraint>
        </Association>

System.Data.Metadata.Edm.AssociationSet as2 = (System.Data.Metadata.Edm.AssociationSet)ac.ElementAt(i).EntitySet;

as2.ElementType.ReferentialConstraints

System.Data.Metadata.Edm.AssociationSet at = (System.Data.Metadata.Edm.AssociationSet)ac.ElementAt(i).EntitySet;
var astype = oc.MetadataWorkspace.GetItems<System.Data.Metadata.Edm.AssociationType>(System.Data.Metadata.Edm.DataSpace.SSpace)
.Where(elem => elem.Name == at.ElementType.Name).First();

string From_Role = astype.ReferentialConstraints[0].FromRole.Name;
string From_Field = astype.ReferentialConstraints[0].FromProperties.First().Name;
string To_Role = astype.ReferentialConstraints[0].ToRole.Name;
string To_Field = astype.ReferentialConstraints[0].ToProperties.First().Name;