Jiras breaking coprocessor compatibility should be marked with component ' Coprocessor', and label 'incompatible'.
Close to releasing 2.0, we should go through all such jiras and write down steps for migrating coprocessor easily.
The idea is, it might be very hard to fix coprocessor breakages by reverse engineering errors, but will be easier we suggest easiest way to fix breakages resulting from each individual incompatible change.
HBASE-17312 is incompatible change. It'll result in 100s of errors because BaseXXXObserver classes are gone and will probably result in a lot of confusion, but if we explicitly mention the fix which is just one line change - replace "Foo extends BaseXXXObserver" with "Foo implements XXXObserver" - it makes it very easy.