reshardCollection doesn't preserve "allowMigrations". It sets "allowMigrations" to false when resharding starts and unsets it after resharding commits.. This turns out to be harmless because of the setAllowMigrations command currently sets "permitMigrations" not "allowMigrations" so the user "permitMigrations" setting is untouched by resharding. However, we need to make reshardColletion preserve "permitMigrations" or "allowMigrations" when we merge the fields in SERVER-61033.