No source code incompatibilities, i.e., the no incompatible changes in the language OT/J nor changes in the API of OTDT.
With the maturing of the new weaving scheme (OTDRE), two different bytecode formats exist, that cannot be mixed with each other. This implies, that class files for sources that utilize OT/J concepts, which where compiled for the old weaving scheme (OTRE), cannot be used when targetting the new runtime environment.
The format is recorded as a bytecode attribute in the class files. Compiler and runtime environment will detect any illegal mixing of class files of different formats.
It is planned to keep supporting the old bytecode format, provided that the BCEL based weaver (OTRE) can be upgraded in time for the release as to fully support Java 8 bytecode. (no version of BCEL capable of Java 8 bytecode has been released in time for Neon)
The BCEL based weaver (OTRE) is still shipped but its development is currently on hold until a Java 8 capable version of BCEL is available.