Welcome to the forum @Josh_Rodarte!
There’s a detailed mLab Guide to Migrating to Atlas which should cover any questions related to migration.
Some of the migration outcomes depend on the type of clusters you are migrating from/to (for example, migrating between shared or dedicated clusters) but the general approach is to allow you to test and confirm your cutover to Atlas without removing any existing data from mLab.
Once you have finalised your migration to Atlas, you need to explicitly Delete the source mLab deployment to remove the data and stop any recurring charges.
If you are live migrating to a dedicated-tier Atlas cluster (M10 and above), see Can I perform a trial/test Live Migration? and related FAQs in the migration guide.