Originally posted by Christian Haselgrove: As far as how to state the license and the copyright that is all fine. The copyright statement has to be dated because copyright expires with some delay (different between juridictions) -- typically 60 to 100 years. That's why you also need to update the date. Often you get something like

(c) 2006-2011 Michael Hanke 

As far a changing the license, the copyright owner has super-powers. The copyright holder can arbitrily change the license at any point. It is also possible to dual-license code (open-source and commercial license, see Qt some years ago). For dual-licensing the copyright holder can also change licenses that are incompatible to each other. Anything goes.  Of course with multiple copyright holder that only applies to the respectives pieces of code one owns. That's why it is important to note that in each file.

If you are not the copyright holder you can still take some code and change the license -- IF the original license permits it. For example, I can always take BSD licensed code and relicense it under the GPL -- as long as I appropriately list the original copyright holders. I cannot, however, take GPL code and relicense it under a closed-source, or even more permissive license.

Regarding how the license change is done: You get the permission of all copyright holders and change the license information in the code, while keeping the copyright statements intact. That's it.