Hello Philippe,
Well, if I understand correctly you have done the migration 8.4.0.X -> 8.5.0.X.
And the management wants a "parachute" so if there are problems they can go back to the old state.
So the best thing is using the last 8.4.0.X backup you had (you did a last backup, did you?)
Restore it, and re-archive all the data since the beginning of the migration.
For TSM, that would be probably a problem, since the object are still in it, and it will probably complain that some objects are already archived...
So in both case, either changing the database structure or restoring the old database and reload everything since then...
will be a pain.
I you have not done the migration yet, but you are looking for a "what do we do if the migration doesn't work", then instead of playing around trying to modify back the tables themselves... the restore of the old database (the ARS% tables) is your best friend... and I would not go to any other road.... too risky, and you won't get support anyway (well maybe not no support, but probably it will difficult to find someone who did it, except by try and errors... so in all cases I consider this road as too risky for a production environment)...
I hope that helps a little bit...
Sincerely yours,
Alessandro
