The Java bindings were revised because (a) the old ones had significantly bit-rotted, (b) we wanted to improve performance by making the code truly a "binding" instead of just a wrapper around MPI calls, and (c) we wanted to extend coverage to all of the current MPI standard. Hence the changes.
The bindings are now stable, which is why they are now coming to a release series. Things in our devel trunk are not guaranteed stable. Note that things in a "feature" series (one that has an odd number in the second part - e.g., 1.7) are not necessarily fully stable as they can undergo changes prior to inclusion in a "super-stable" series (an even number in the second part). At that time, only bug fixes are allowed.
That said, we don't anticipate changes to the Java binding APIs.
The new bindings are documented via javadocs and man pages, which should help in the transition. If you have specific questions, I suspect some of the folks more directly involved will try to help.