Currently there's no way how to reassociate orhapned unit back to repo. Workaround solution: Upload again orphaned content directly from pulp content directory. However access to pulp content directory is needed.
Currently, if a user unassociates a unit from a repo and the unit is not in any other repos, it becomes orphaned (correct behavior). However, there is no way to fix or undo this operation via pulp-admin without re-uploading the unit again. One way to fix this may be to be able to list orphans by unit ID and then re-associate via a new pulp-admin command.
This is not issue of pulp-admin. Pulp-admin still missing some crucial functionality (like associate distributor with repo) therefore we have no other choice but use REST for things that cannot be done with PA. I'm not sure, but I don't think it's possible associate orphaned unit by unit ID. Not with pulp-admin and neighter via REST API. But if you know how to do this, please let me know.
This is not currently possible via the either the REST interface or pulp-admin unfortunately. We'd have to add a new API in order to do this type of association without re-uploading the unit.
Moved to https://pulp.plan.io/issues/268