Scala: collecting updates/changes of immutable state?

This is basically the same as views and is a type of lazy evaluation; this type of strategy is more or less the default in Haskell, and is used in Scala a fair bit (see e.g. MapValues on maps, grouped on collections, pretty much anything on Iterator or Stream that returns another Iterator or Stream, etc.). It is a proven strategy to avoid extra work in the right context.

I tried to write a (quite rough) test for timing performances on your case class copy operation.

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