Welcome to the MongoDB Community! 
I have read your post and you want to change your schema. MongoDB offers several resources and guides for best practices for schema designing. Here are,
The example problem that you’ve posted is how to update the field type, for that I can suggest you use the aggregation operations .
For your specific case: to update date_added field type from string to date, you can use $convert and $set aggregation operators in specific order stages and you can also verify field type by using $type.
MongoDB offers flexibility to perform aggregation operations on multiple documents consisting of a rich variety of operators. I suggest you to refer this documentation and you can smoothly manage your schema as well as the database 
Best,
Shreya