let's say a moodle instance exists that uses db sessions. and then this same instance is going to change where sessions are kept to outside of the database. this change is made in the existing instance by updating the config.php file with the new specifics of the session store.

this is in the docs:
https://docs.moodle.org/33/en/Session_handling

but once this is done, $CFG->dbsessions will remain selected as the admin page essentially shuts a user out of being able to change this with admin settings web page.

i suggest a patch to the code in admin/settings/server.php around line 40. here's the current code:

{{// "sessionhandling" settingpage
$temp = new admin_settingpage('sessionhandling', new lang_string('sessionhandling', 'admin'));
if (empty($CFG->session_handler_class) and $DB->session_lock_supported()) {
$temp->add(new admin_setting_configcheckbox('dbsessions', new lang_string('dbsessions', 'admin'), new lang_string('configdbsessions', 'admin'), 0));
}

to fix this, i would add a check to see if $CFG->dbsessions is already set. if so, then i'd display the checkbox.