It may seem to be a bit silly question, but I couldn't find a strict definition that would address my specific situation (I appreciate I'm not the first to experience it though).
There's a library licensed under Apache 2.0 for which I'm going to make improvements. My approach is not modifying the original code of the library but rather make a some wrappers/adaptors and also some classes that may copy up to 75% of the functionality of the original library, but are in fact independent.
The question is basically whether these partially copy-pasted classes are considered modifications of the original code. They are not replacing the original files, but live independently from it in the new library I'm working on. When someone wants to use my library, they will need to use the original unmodified library too. However my library is still based on the original code and my gut feeling suggests I should keep the original copyright notice to meet Apache 2.0 license's requirements.