I'm planning to fork an open-source project which is licensed under Apache License v2. Most of the existing sources have a copyright notice in the heade, Copyright bla bla, Inc.
If I change some these sources in my fork (even if only marginally), what happens to the copyright? Will it become a joined copyright notice like "Copyright bla bla, Inc and Thomas"?
Disclaimer: I'm not a lawyer, just a software developer. This is just my own opinion, in each concrete case if you need legal help you must talk to a legal team on your behalf.
If I change some these sources in my fork (even if only marginally), what happens to the copyright?
The copyright remains for it's code. If you remove the code, there is no copyright any longer. If you modify the code, it's like you already assume, there is copyright for those lines of code, and copyright for the new lines of code.
Will it become a joined copyright notice like "Copyright bla bla, Inc and Thomas"?
If there is a notice per file: If you don't change the file, keep it as-is including the copyright header.
If you make changes to the file, normally the new copyright is put on top of the file, then making it visible that this is based on the previous code and then the original license plate follows. Most licenses require that you do not change the original copyright/license notice, this is important otherwise you normally loose the rights to make use of the software (e.g. fork it).
The important part is that it's clear which code falls under which copyright, so you have it documented on your own as well. With the method described, you know which files you have modified (and which files remain unmodified).
Use version control software so you can follow any changes.
A good article, even if GPL does not apply for your fork, is: Maintaining Permissive-Licensed Files in a GPL-Licensed Project: Guidelines for Developers.