Moving database collections from one server to another but in different schema

Hi All,
I have a live database(A) in mongodb which is around 2TB in size. I also have databases B and C present on another server.
The database A has 15 collections . Now i want to migrate the collections along with live data from database A to B and C with a different schema structure.
what is the best possible way in which i can achieve this with minimum downtime ??