i have following design in mind, but dunno is it appropriate or any better way to do that:

i want to make two classes: Payment & CompletedPayment
in which CompletedPayment extends Payment
with more specific attributes

i would use the <joined-subclass> to include the definition of CompletedPayment inside Payment

my system would first create a Payment object and save into DB

somewhen later, upon request, the system would load the Payment obj, cast (or copy or else?) it as CompletedPayment, add the details, then saveOrUpdate it back to DB.
so that the CompletedPayment details is saved into the joined table, while updating the Payment table...

It seems im marrowing the class of a persistent obj. Im doubt can i achieve that or should use another approach.

any suggestions?

nop, delete and create a new CompletedPayment based on the old payment properties