Hi, Thanks to Max, the instance migration when changing a class with existing instances is now aware of slots. e.g., you can Object subclass: #A slots: { #iv => TestSlot } classVariables: { } category: ‘Playground' do A new, change iv, then change the definition to a normal instance var: Object subclass: #A slots: { #iv } classVariables: { } category: ‘Playground' ==> instance is migrated while keeping the value. TODO (to have a first complete version) -> hidden slots (slice needs another pass) -> build hook -> call #initialize: on all Slots after #new when requested. Marcus