I've put my project to Google code by using the GPL 3 License. Can I change it to Apache 2.0 ? Including all copyrights in sources.
Updated: I know how to do that. I'd like to know about issues that can happen. Or I can just change it and don't worry about that?
You can certainly change the license on the Google Code front page for your project by using the Administer tab. This of course will not change any license information in the source itself, which is a good reason for not putting such information in the source.
If you are asking "is it OK to change the license", then if you are the copyright holder, yes it is. Doing so will not however revoke the previous license for existing users of your code.
Go to administer->project summary and change it there. Copyrights in sources you need to update yourself.