Well, I'd say this is less a GPL specific question but more about Copyright in general.
Let's make it scenario that you make copyrightable changes to the work. You are allowed to do that because GPLv3 is a Free Software License which grants you a right to copy and modfiy the program.
So let's say so far you did the changes only in code, not inside (copyright) commments or documentation files with that code.
That would mean the original copyright line is there for the whole work in the one or other form:
Copyright 2012 Jane-Original-Holder Toe
As you made copyrightable changes to the work this copyright line is not correct any longer. First of all because you did the changes this year 2013, the year needs to be updated.
Also Jane-Original-Holder Toe does not hold the copyright for the whole work alone. The information is wrong.
As you should not provide wrong copyright information (depending on where you live this is more or less controlled by law what that means in concrete), you should consider to reflect the changes in copyright (I can only suggest you that as a software developer).
So marking the modification is more or less a requirement by law, and not by the GPL v3 in specific.
One easy way to keep documentation of who wrote and committed which change when to a codebase is to use a SCM / RCS which allows you to specify committer and author, for example Git.
Also for the existing history. And also as Martin Beckett wrote, consider to give changes back upstream. Live is too short to develop alone.
Turns out, there is a lot of related material which you didn't reference in your question, so see as well:
and most likely some more.