In JLS chapter 13 on binary compatibility, there is a section discussing compatible transformations for packages and modules (https://docs.oracle.com/javase/specs/jls/se9/html/jls-13.html#jls-13.3).

An issue that came before the CSR for review (JDK-8198582) involved moving some types in an existing to a new module that was in turn exported from the original module. This is also a compatible transformation and since it is expected to occur again in the future, it would be reassuring if the transformation were explicitly discussed in JLS chapter 13.