Constraints could be added or deleted using following syntax:
ALTER TABLE [db].name [ON CLUSTER cluster] ADD CONSTRAINT constraint_name CHECK expression;
ALTER TABLE [db].name [ON CLUSTER cluster] DROP CONSTRAINT constraint_name;
See more on constraints.
Queries will add or remove metadata about constraints from table, so they are processed immediately.
Constraint check will not be executed on existing data if it was added.
All changes on replicated tables are broadcasted to ZooKeeper and will be applied on other replicas as well.