BIggest issue with forking cryptocurrencies is that who can legally change the license that the original creator inserted to the code, and if it is changed to conflicting one, is it still legally binding.
As I see it, you can replace full directories from source code and use compatible license in directory that replaces the removed directory.
If license in new code is incompatible with license of the original code in the root directory of the source code, it is not legally binding.
Using two licenses in same source directory is only allowed if two binary files don't touch each other as in neither is linked against the other.