immutable_fields
Adds EffectiveId and IsMerged, plus Type on update. Merging via REST2 goes through the MergeInto field of an update, which is rights-checked.
Type stays settable on create, as that is the only way REST2 clients have to make a reminder. Setting it afterwards is another matter: searches match Type = 'ticket', so re-typing an existing ticket would drop it out of every result.