The change in terminology membership operators from terminology reference elements to expressions is a potentially backwards-incompatible change in the ELM, because if the ELM is serialized without a type element (the default for concretely-specified elements), then the deserialization into the schema with an abstractly-specified element will fail. Suggest instead of changing the type of the terminology reference element to Expression, adding a new element for the terminology when specified as an expression.

Specifically:

Change InValueSet.valueSet back to a ValueSetRef
Change AnyInValueSet.valueSet back to a ValueSetRef
Change InCodeSystem.codeSystem back to a CodeSystemRef
Change AnyInCodeSystem.codeSystem back to a CodeSystemRef
Add an InValueSet.valueSetEx element of type Expression
Add an AnyInValueSet.valueSetEx element of type Expression
Add an InCodeSystem.codeSystemEx element of type Expression
Add an AnyInCodeSystem.codeSystemEx element of type Expression