It's been a while since I've played with mongo, but you could run an out-of-band task to run through all existing data, transform, then push into a new DB / ins

It's been a while since I've played with mongo, but you could run an out-of-band task to run through all existing data, transform, then push into a new DB / instance. Your existing server would continue, you have a new endpoint to test with updated application code, deployment can then be a standard blue-green approach.