Change subclass at runtime ?

I have inheritance in my application and sometimes I have to change the 
subclass of a record. How to do that ?

It is vertical inheritance so in theory, you have to change the class 
discriminating attribute in the database but if you do this at run 
time, the class may be already instanciated... do I have to refault the 
object ? and what about relationship ?

One of my solution to this problem is to create a new object of the 
class we want to change to, copy all values of the properties of the 
old object to the new one and then delete the old object. This is a 
solution but it is much more complicated than changing the 
discriminating attribute...

Anybody having already found something for this ?

Dominique