Updating just a single content field from a plugin
I don't have much experience using tasks, but I often write plugin methods to move data around. Typically, I just use the EntriesService saveEntry or RelationService saveRelation as needed.

Here is a sample from a recent data move:

    // Assign category and save entries
    foreach($unassignedEntries as $entry) {

        $success = craft()-&gt;relations-&gt;saveRelations($field, $entry, array($categoryId));

        if (!$success) {
            LogicPlugin::log('Couldn’t update entry "'.$entry-&gt;title.'"', LogLevel::Error);
        }
        else {
            LogicPlugin::log('Successfully updated entry "'.$entry-&gt;title.'"', LogLevel::Info);
        }
    }