It would appear the only reason to throw away the set of snapshots used for readConcern is if the local node enters ROLLBACK:
https://github.com/mongodb/mongo/blob/f8dadcefeef7e738d7063ae2df57b9c856b402da/src/mongo/db/repl/replication_coordinator_impl.cpp#L2518

Replica-set reconfigs would seem to be another way to change the conditions needed to fulfill readConcern majority. For example, in a 5-member replica-set where 2 members are non-voting requires only 2 of the voting members to fulfill a majority readConcern. Reconfig to allow the other 2 members to vote means the set should now demand 3 members to achieve readConcern majority. However, the existing 'verified' snapshot on all members would continue to be used for this purpose despite it (not necessarily) fulfilling the current requirements.