f2[l_, n_] := n l;

Clear[x, y, z]
masterl = {x, y, z};

xx = 3; yy = 4;
l2 = masterl /. {x -> xx, y -> yy}

(* {3, 4, z} *)

f2[masterl, 5]

(* {5 x, 5 y, 5 z} *)