Let’s say that we deliver the new shardCollection in 7.1.  Thus, when we are upgrading from 7.0 to 7.1, as part of the setFCV process we have to drain ‘shardCollections’ using the old implementation and start using the new one. Symmetrically, on FCV downgrade from 7.1 to 7.0 we have to drain shardCollections using the new implementation.
We also need FCV stability when we decide which coordinator we are going to spawn.