Once a collection has been created, use the collMod command to modify the validator for a collection. See Database Commands in the Node.js driver documentation for a general example of how to call a database command like collMod.
I couldn’t find an obvious example of modifying schema validation in the Node.js driver docs, so I created an improvement suggestion in the issue tracker which you can upvote & watch: NODE-2511: Add example for changing validator on an existing collection.
Note: the $jsonSchema query operator is also useful to test schema validation changes against existing documents. For an example using the mongo shell, see: How to find all invalid document based on jsonSchema validator?.
Regards,
Stennie