Re: Objects with varying classes

In general, the only way to change the `__class__` of a Persistent object 
is to update *all* references to it. For references in a BTree, that may 
mean updating the tree itself (if small) or the individual bucket. There's 
further discussion of this 
here: https://github.com/zopefoundation/BTrees/issues/72

Because finding and changing all references is usually difficult (even one 
missed reference can break everything), this is not usually recommended for 
applications (migration tools can be written to do this by crawling through 
the entire database object-by-object, but that's rarely practical for an 
application). This is one way that interfaces can be more useful than 
classes: dynamically changing the interfaces an object provides with 
`interface.alsoProvides(object, IFoo)` or 
`interface.noLongerProvides(object, IFoo)` propagates automatically to all 
references to the object.

HTH,
Jason

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