Yes you need to change the codebehind file and its namespace of each classes.
The parser issues is because you dint change your code behind file
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Forms.aspx.cs" Inherits="Dhaka_Education_Board.Forms" %>
if you have changed this and if u get the error still, then go to your class file and check its namespace or class name