Constraints could be added or deleted using following syntax:
ALTER TABLE [db].name ADD CONSTRAINT constraint_name CHECK expression;
ALTER TABLE [db].name 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.