Adding cascading deletion to an existing constraint

No. Changing the constraint isn't possible, the option available is to remove it and create it again

ALTER TABLE [TARGETTABLE] DROP CONSTRAINT [KEYNAME]

A foreign key constraint

Alter Table FirstTable Add Constraint [KEYNAME]
Foreign Key (Field) References SecondTable (Field) On Update Cascade On Delete Cascade

A primary key constraint

Alter Table Target add constraint [Main Key]
Primary key(Field1,Field2,.....)

Constraints cannot ever be changed, although you can remove them and create replacements.

Take a look here

ALTER TABLE target_table
DROP CONSTRAINT CURRENTPLAN_FK;

then create the replacement using ON DELETE CASCADE this way

ALTER TABLE target_table ADD CONSTRAINT CURRENTPLAN_FK
FOREIGN KEY(CurrentPlanCode) REFERENCES PLAN(CurrentPlanCode)
ON DELETE CASCADE;

hope that help