The generic function
update-instance-for-different-class
behaves as specified for ANSI Common Lisp.
During the operation of updating the instance, including the call to
update-instance-for-different-class
, the redefined instance is locked against access. Any other process that tries to access the instance will hang until the operation finishes. Therefore your methods must avoid doing anything that may wait for another process which may access the instance, as this would cause a deadlock.