" Older versions of CQL such as STU 2 can be used provided they work with FHIR R4."

Who can do that?  Presumably EHRs and SMART apps have to support the full capabilities of the current release, otherwise we won't have interoperability.  Are there any breaking changes between STU 2 or prior releases and the current one?  If so, what happens if someone uses and older version?  Do we understand how systems will determine what version of CQL they're seeing when it's embedded in an extension or found in a library?  We can't support multiple versions unless we know what version we're seeing - and supporting multiple versions add cost.  Is the cost justified?