I think you are looking for something like this:

package ModelicaTest30
  model M
  end M;
  // model1
  model M1
  extends M;
  parameter Real p1 = 1 "p1";
    Real y "y";
  equation
    y = p1 * time;
  end M1;

  // model2
  model M2
  extends M;
  parameter Real p2 = 1 "p2";
    Real y "y";
  equation
    y = p2 * time + 23.145;
  end M2;

  // system
  model SYS
  parameter Real a = 10 "a";
    parameter Real b = -2 "b";
    Real z "z";
    replaceable M1 m constrainedby M annotation(
      choices(choice(redeclare M1 m(p1 = 10) "Default"), choice(redeclare M2 m(p2 = 20) "Variant")));
  equation
    z = a * m.y + b;
  end SYS;

  // Test
model Essai
//SYS sys1 (redeclare M1 m (p1 = 2)) ;
//SYS sys1 (redeclare M2 m (p2 = 4)) ;
    SYS sys (redeclare M2 m (p2 = 3.254))  annotation(
      Placement(visible = true, transformation(origin = {4, 6}, extent = {{-18, -18}, {18, 18}}, rotation = 0)));
end Essai;
end ModelicaTest30;

You can choose between the compatible models, and parameters ,in SYS, but you can't edit graphically the parameters in m. You can specify them manually.