https://askubuntu.com/questions/728620/can-queued-messages-be-moved-between-postfix-servers/#answer-1155503

chown -R postfix:postdrop /mnt/old_var/spool/postfix/maildrop
chown -R postfix:postfix  /mnt/old_var/spool/postfix/defer*

mv /mnt/old_var/spool/postfix/maildrop/* /var/spool/postfix/maildrop/

cd /mnt/old_var/spool/postfix/

find defer -type f -printf "%P\n" \
| while read f; do
    for d in defer deferred; do
      mv -v $d/$f /var/spool/postfix/$d/$f
    done
done

or in 2 lines:

cd /mnt/old_var/spool/postfix/
find defer -type f -printf "%P\n" | while read f; do for d in defer deferred; do mv -v $d/$f /var/spool/postfix/$d/$f; done; done