As of MongoDB 4.0, we fail to start up unless all collections have UUIDs to account for 3.4 to 3.6 UUID assignment bugs. To reduce the number of situations in which users have to set the FCV back to 3.4 and to 3.6 again to re-assign UUIDs, non-replicated collections automatically get assigned UUIDs on start up. All such UUID fix-up code should be removed in 4.2, as it is risky to keep around. Skip-level upgrades from 3.6 to 4.2 are prohibited, so it should be safe to isolate UUID fix-up code to MongoDB 4.0.