SQL & PL/SQL

hi my table having the check constraint. but now i want to change the values in that constraint.
is it possible to change the check constraint values. and with out disable or drop my constraint. i dont want to change the constraint name also.
i am expecting answer with out drop or disable the existing constraint
now i will give you my existing constraint and proposed constraint syntax.

my existing constraint Syntax is :
CONSTRAINT CONS_MRANTYPE
CHECK (MRAN_TYPE IN ('SP','JW','SD','SC','OT')

my proposed constraint syntax is :
CONSTRAINT CONS_MRANTYPE
CHECK (MRAN_TYPE IN ('SP','JW','SD','SC','OT','JR')

Thanks
Indra