Hi Mary,
We have this problem at South Devon College. We upgraded our assignments with the Assignment upgrade tool in July and none of the previous Offline Assignments are displaying their grades and feedback comments when the teacher or student opens the assignment. We were on 2.3.1 (build: 20120719 ) at the time.
Just upgraded to 2.3.2 and this problem has been fixed however the previously upgraded assignments are still missing their previous feedback or grade.
I've not been able to locate the previous grade but I have discovered that if the student views their User Report in Grades then the feedback is displayed (no grade still) and if the teacher clicks on the grade cell in Gradebook the tooltip that appears contains the feedback but this is not a user-friendly way to view the feedback and with long feedback not all of it is displayed.
This feedback needs to be made available in the View/grade all submissions page and the grade restored if possible.
I have also updated your tracker http://tracker.moodle.org/browse/MDLSITE-1816