In R4, MedicationAdministration has 'request', which covers part of the basedOn semantics.  It's possible to pre-adopt the R5 MedicationAdministration.basedOn with the standard pre-adoption mechanism.  We've recommended to pharmacy that MedicationAdministration.basedOn should extend to include MedicationRequest and ServiceRequest in addition to CarePlan. Given that there are other ways to meet the R4 requirement, we don't feel this is an appropriate change to include as a technical correction. Pharmacy discussed this and will be making a separate Jira issue to discuss a change to basedOn and/or request for R5.  When that happens, this use-case be met using the R5 pre-adoption extension mechanism.  We agree with the rationale for not making this change to the event-basedOn extension because currently the semantics are split between basedOn and request.