Experience has shown that sometimes you'll get into problems using "slot-value" or variables introduced with "with-slots". It may appear the the EQ-ness of objects in not preserved because the compiler was "smart enough" to optimize these features, so for instance: if it's possible to access the same object (EQ) with another symbol, the compiler can return different slot-value's for the original or the other symbol. Even if they are EQ! This can be very confusing.
For instance:
'((cl :mode :display)
(defvar *A* (make-instance 'bar))
(defmethod foo ((x bar))
(change-class x 'baz)
(format t "~&from x: ~S from *A*: ~S"
(slot-value x 'quux)
(slot-value *A* 'quux)))
(foo *A*))
Can produce output like: "from x: 1 from *A*: 2", depending
on the compiler and/or optimization qualities.
To solve this just call a helper function just after
changing the class. The compiler will not optimize over function calls
so the slot-value and with-slots features work again.
programming tips