You can't enable and disable
improved_mboxlist_sort on a live
system. You need to dump and load the necessary database after stopping
and before starting the master process. Rename the original mailboxes.db
out of the way between dumping the old and loading the new.
Dumping the mailboxes.db file
ctl_mboxlist -d > /var/tmp/mailboxes.txt
mv mailboxes.db mailboxes.db.orig
ctl_mboxlist -u < /var/tmp/mailboxes.txt
If your subscription databases are not in flat files you need to do something similar. Each user will have his own subscription file. Do the following for each subscription file.
cyr_dbtool -C $file skiplist show > $file.TXT
cyr_dbtool -n $file skiplist set < $file.TXT
The above fragments will overwrite the original file. So you could redirect to a temporary file and overwrite the database if the import succeeds.