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A transformation seems like it would affect both comparison and update.

You would probably need to create your own (dynamic) provider to achieve this behaviour.

Can you specify the provider per resource, or would we need to do something top level?

Also, would we also need to subclass the resource (c#) or is there a "property bag" for extensibility properties

You can specify the provider per resource, using the _provider_ opt. I always do.

However to change a resource's provider without destroying and recreating the stack, I think you need to manually edit the stack.