we've been using the contributed "PDF Feedback" plugin for some assignments in our Moodle 2.5 installation.
We're now going to migrate to Moodle 2.7 and will likely be using the "Annotate PDF" plugin which, as far as I understand, must have started out from the same code but has now ended up in Moodle Core.
Is there any easy way of migrating existing data from the one plugin to the other? (If it were just for the student's initial submissions and for the final annotated PDF files, read-only, that would be enough.)
No, there is no migration code available. It probably wouldn't be that hard to write, as mostly it would a straight copy of data and files from one location to another (I haven't looked in great detail at the new data structures, but much of the back-end code is the same as my original).
Unfortunately, I'm unlikely to have any spare time to write this code, so you'd either need to find someone whow does have the time, or see if there is some funding available to pay for it.