Existing Wording
Most code systems evolve over time, due to corrections, clarifications, and changes to approach or underlying knowledge or reality. If these changes lead to the meanings of existing codes changing significantly, then the interpretation of the code system becomes version dependent. This significantly complicates implementation based on the code system, to the point where it is not clear that safety can be assured, so SHOULD be avoided whenever possible.

Proposed Wording
This significantly complicates implementation based on the code system, to the point where it is not clear that safety can be assured, and MUST be avoided.

Comments
From Cimino's "Desiderata for Controlled Medical Vocabularies in the Twenty-First Century", the priniple of concept permanance states that "the meaning of a concept, once created, is inviolate. Its preferred name may evolve, and it may be flagged inactive or archaic, but its meaning must remain".

Grahame's Comments
we can't make that rule. Unless you want to ban LOINC, for instance. We're not saying that it's not good, but the reality is, not everyone does the right thing all the time. (or no one does)

Disposition
Not persuasive

Disposition Comment
While it would be nice if all terminologies were well-behaved, the reality is they aren't - and HL7 doesn't have the power to make them. Thus the FHIR standard has to provide mechanisms to deal with reality.