Where the original parties to the suit challenging union fees leave covered employment during the pendency of the appeal and can therefore no longer receive the sought equitable and injunctive relief, the case is moot; it cannot be converted into an action for damages, and an organizational plaintiff cannot be joined to preserve standing.
The remedy is dismissal without a vacatur of the earlier decision on the merits.
http://cdn.ca9.uscourts.gov/datastore/opinions/2018/06/11/16-55768.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.