Hello, guys i want to do a simple thing, which can be expressed by following: oldClass replaceWith: newClass migrateInstances: [:inst | ... ] In my case, first, i installing a WeakFinalizationRegistry class, and then want to replace an old WeakRegistry class with it, silently and painlessly. I end up with following script: | old new oldClass newClass | old := OrderedCollection new. new := OrderedCollection new. "migrate instances" WeakRegistry allInstancesDo: [:registry | | newr | old add: registry. newr := WeakFinalizationRegistry basicNew initialize. registry migrateTo: newr. new add: newr ]. old asArray elementsForwardIdentityTo: new asArray. "replace the class" oldClass := WeakRegistry. newClass := WeakFinalizationRegistry. Smalltalk forgetClass: #WeakFinalizationRegistry logged: false. newClass superclass removeSubclass: newClass. newClass setName: #WeakRegistry. oldClass becomeForward: newClass. -- it looks a bit hackish, and i wonder how to do that in more 'standard' way. Initially replacing classes was: oldClass removeFromSystem. newClass rename: #WeakRegistry the problem with that, is that all compiled methods , which was referring to old class through association, still using it, and to get rid of them i have to recompile them. -- Best regards, Igor Stasenko AKA sig.