Nested Master Page and Master Page

I have a parent MasterPage (Main.master) and a child NestedMasterPage
In the NestedMasterPage i have assigned the MasterPageFile property in the html code like below
<%@ Master Language="C#" AutoEventWireup="true" MasterPageFile="~/Themes/ABC/Main.Master" CodeBehind="User_Main.master.cs" Inherits="ProjectABC.UI.Pages.MasterPages.User_Main" %> Now in the code behind of the NestedMasterPage i am trying to change the MasterPageFile property to some other MasterPage but its not work ...