Can I add an enabled-by-default lightswitch field to existing entries?
If there was no default set beforehand, then your previous elements will have a value of null for that field. You should be able to include those in your "on" queries by querying for that null value as well as the "on" value in your templates:

{% set onEntries = craft.entries.yourLightswitch([null, '1']).find %}


This won't actually change the value of your previous elements though. One way to do that would be using a SQL query against the craft_content table to update rows (elements) that have null in that column:

UPDATE `craft_content` SET `field_yourLightswitch` = 1 WHERE `field_yourLightswitch` IS NULL;