Enabling improved_mboxlist_sort

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.