Hi,
yes, you can actually exchange the headefiles. But I don't know if in the depths of the IDE old names of the bit definitions are used. I work only with current bit definitions. For this I wanted to create a compromise with forward compatibility that otherwise doesn't bother anyone. Is only meant for people who have only the megaAVR boards package installed. Who installs the MegaCoreX package is already advanced anyway. 
My concern is that I don't have to ask anyone to install any other packages for my examples. I can recommend this, but it should not be mandatory.
I myself know how to handle additional packages and new toolchains.