Casting/converting generic list types

If I have List<A> where all of the elements are of type B.  How do I get
List<B> from List<A>?  Do I have to iterate over List<A> and add each
element to List<B>?

I've tried using "as List<B>" and "(List<B>)" both to no avail.
List<T>.AddRange() must be of the same type T.

ConvertAll() is an O(n) operation, and does a conversion which I don't
really need.

I have this situation occur fairly frequently because A is an  abstract
superclass of a number of different classes and it has a method to return a
List of objects of a subclass, but that list is always homogeneous.

Insight appreciated,
-Mont

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