This article is from the Piano General Topics FAQ, by Isako Hoshino firstname.lastname@example.org with numerous contributions by others.
Regarding derivative versions of previous works, including
musical arrangements, adaptations, revised or newly edited
editions: the derivative works are independently copyright-
able, and the copyrights of such works do not affect or
extend the protection, if any, of the underlying work.
I would presume that the concept of a derivative work applies
to a musical work which has been edited, and that any
additions or changes due to editing is what is being
copyrighted, but I have not run across specifics in this
...in other words...
If you want to make a simplified edition of something--you
have to use music in the public domain or you have to get
permission from the copyright holder. It is sometimes quite
a search to find out and to secure permission. But it must