Question Regarding Copyright Issue For An Open Source Project

[Yan Cheng Cheok]

Hi,

I am an author of an Open Source project, which is released under GPL2. I start to work on it alone for 2 years. 

In every of my source code files, I attach the following information on the top of it.

/*
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or (at
 * your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 *
 * Copyright (C) 2009 Yan Cheng Cheok <yccheok at yahoo.com>
 */

Now, the project is getting attention from the public. There are increasing number of programmers are joining in. I was wondering :

(1) When there is a programmer modify my original source code file, do I need to add his name in the copyright section. But, with the increasing number of programmer, isn't that will keep the header almost unreadable? 
For example :

 * Copyright (C) 2009 Yan Cheng Cheok <yccheok at yahoo.com>, John <john at gmail.com>

(2) If a programmer add a new source code file, the source code copyright shall belong to whom? Me? Or him?

(3) If there is a mixed copyright source code in a project, say, 

A.c, B.c, C.c source code file is copyrighted Yan Cheng Cheok
D.c, E.c, F.c source code file is copyrighted John

Will there be an issue? Say, in the future, John decide to switch D.c, E.c, F.c using different license, and Yan Cheng Cheok doesn't agree with it...... Who will have a final say then? 

In order to avoid this type of conflict, shall I enforce all commited source code shall be copyright under me? But, I also do not want the new developer feel that his work is not being credited properly. 

(4) Is there really to have <year> in the copyright information? If I put 2009, does that mean in 2010, I am no longer holding the copyright?

Thanks and Regards
Yan Cheng Cheok