Syncing of two mongo databases

We are about to migrate from one mongo infrastructure to another. In the mean time, is there a way, while we test the new setup, that we can sync the two databases?

I imagine a scenario, where while we test the new infrastructure, the old infrastructure will sync with the new database, so they are aligned data-wise, and if anything should go wrong, we could simply change the mongo URI to the old database.

What are my options?