On May 21, 2007, at 9:08 PM, Alan Lovejoy wrote:

> ...
> Of course, no OS I know of has the equivalent of ClassBuilders that  
> can
> transparently migrate instances of old (versions of) classes to be  
> instances
> of new (versions of) classes.  ...

Well, as long as you're asking...

http://www.symbolics.com/Genera-1.htm                     (note  
feature bullets)

How this was done, exactly, for class defs:
http://www.lispworks.com/documentation/HyperSpec/Body/07_b.htm
http://lisp.org/mop/concepts.html#reinit-class

If I recall, there was an implementation of Objective-C that also  
updated existing instances when you changed class shape, but I never  
(knowingly) used it.