This depends on on the language of course. In Common Lisp, a generics function called REINITIALIZE-INSTANCE is called. It's possibly to implement a method for t

This depends on on the language of course. In Common Lisp, a generics function called REINITIALIZE-INSTANCE is called. It's possibly to implement a method for this that updates the instance based on the new definition. By default, any slots that have been added will be unbound. Attempting to access them will raise an error which will drop you in the debugger. From there you can then fix the instance and continue running.