Currently FCV upgrade as well as downgrade for change streams over timeseries collections are problematic.

FCV upgrade:

• change stream opened over system.buckets collection
• FCV upgrade occurs
• getMore() over system.buckets collection is prohibited, failing the stream to observe rename+invalidate events

FCV downgrade:

• change stream opened over timeseries collection nss
• FCV downgrade occurs
• getMore() over change stream cursors is issued, RetryChangeStream exception is thrown as placement history is removed
• failure to reopen a change stream, as timeseries collection nss is a view and change stream over a view are prohibited

Potential solutions:

• Do nothing, accept minor quirks around FCV upgrade/downgrade for change streams over timeseries collections only
• Allow getMore() over buckets collection in kCommandsAllowedToTargetBuckets (ref) 
• Allow opening change streams over view (original namespace) if that namespace is timeseries collection