SDWG page.

Template compatibility requirements

Ability to indicate forward/backward compatibility
- Forward compatibility: An instance that conforms to a new version of a template will also continue to conform to a prior version of the template. (Example: Template X, version 1 constrains observation.code to Y. Version 2 constrains observation.code to Y and also constrains observation.effectiveTime to TS)
- Backward compatibility: An instance that conforms to an old version of a template will also continue to conform to a new version of the template.

From the perspective of the IG constraint

Template referencing is ALWAYS static (e.g. is ALWAYS to a specific version).
Ability to constrain to a particular version of a template.
Different versions retain the same name (suggesting the need for id and setId).
Ability to distinguish a family of templates in a versioned chain from the particular version within a family.

Template referencing is ALWAYS static (e.g. is ALWAYS to a specific version).
An instance needs to be able to claim conformance with a particular version of a template.

CURRENT WORKING ASSUMPTION IS THAT THIS IS THE PREFERRED APPROACH (subject to some refinement): No versioning. Any change results in new templateId. (Assumes the ability to assert various semantic relationships, such as "supercedes", along with "implies" and "contains").
Add version number / version date to template (and allow for inclusion in references).
Assume that in the absence of a version in a reference, the reference is to version 1.