See PHI1433. Currently, objects which support subtypes also always support a "Change Subtype" comment action. An install would prefer that certain subtypes be hard to change, and this action is probably often of little use when subtypes are dissimilar.

We can reasonably make this configurable to improve the UI in cases where some or all subtype changes don't make sense.

This is an advisory change only; the API or Bulk Edit flows can still perform any subtype mutation. (The assumption is that if you're using the API to meddle with the internals, you presumably know what you're doing, and this gives administrators an escape hatch without requiring piles of new granular permissions.)

See also T13248.