I believe that one of the features of a reflink is that it tells you what else it's linked with, so that you can find it to break the COW
if this isn't the case, it should be, for just this reason.
as I understand things, if a file is changed it may break the linking entirely (copying the entire file), or it may break the link partially (still sharing the common parts of the file, but with the differences being separated) at the option of the filesystem