How to apply GNU/GPL to an abandoned project?

[Andrés Felipe Vargas Valencia]

Hello..

My name is Andrés Felipe Vargas, I'm developer and I'm interested in 
continue the work in a project that seems abandoned, the project is a 
extension for Mambo called 404sef that ext is released under the GNU/GPL 
terms, the files of the project are hosted in www.sourceforge.net as the 
project seems abandoned i have requested a take over to SF and they have 
accepted.

Now, of course I want still releasing the exp like GNU/GPL, but how i should 
write the copyright notes if i want to add my work to that project?

should I mark any line of code that i change? or can I change the copyright 
notes?

Thanks in advance for your help

Andrés Felipe Vargas