A possible solution is:
grommunio-admin user query username maildir | grep <ALTER_USER>
grommunio-admin user query username maildir | grep <NEUER_USER>
Determine the mail directories of the two mailboxes and then exchange them with mv
.
Then systemctl restart grommunio-zcore grommunio-http php-fpm
so that the caches are empty.
Use at your own risk.