It is recommended to also read KDE's Licensing Policy.
One can claim copyright on a file after doing significant non-cosmetic changes (for instance, indentation, const or variable name changes are cosmetic changes). Adding a non-trivial function, significant refactoring, adding more than ten lines of code, etc... are changes that give copyright.
If you add copyright a year or more after your last change, and that you do not appear in the annotation of that file, it is important to justify the reason for such an addition.
It is worth to mention, that copyright claim is usually made by the author of a contribution.
Copyright should be removed only in case you are absolutely certain the person mentioned does not hold copyright on the file, it can happen if the line was added by mistake. If such case, you are advised to first contact the author to ask him if is agree, if you don't get an answer or if there is a disagreement, you need to send an email on the developer mailing list asking for others people opinion, and then if an agreement is reached, the copyright line can be removed.
Even if the code written by the person was entirely removed from the file, the copyright line needs to stay in place unless you get explicit permission from the person to remove it, or consult a lawyer.
Annotation is not an efficient way to know if a person hold copyright on a piece of code, only a deep inspection of all the commits can bring sufficient proof.
In all case of copyright removal, it is required to put both the koffice developer list and the author in copy.
Moving code to a different file
It is absolutely not allowed to take code from a file, put it in an other, and put his name on top of the new file. The copyright header of the previous file need to be used in the new one.
Maintaining Permissive-Licensed Files in a GPL-Licensed Project: Guidelines for Developers section 5 is of interest for that policy