Which of the following is a valid change to fields with a lock icon next to their name in a schema?

In Adobe Experience Platform schemas, fields that are represented with a lock icon signify that they are immutable once they are created and cannot be changed in terms of their core attributes. This includes modifications such as renaming the field, deleting it, or marking it as an identity field, all of which would disrupt the structure and integrity of the schema.

Adding a relationship, however, is a valid action for fields that are locked. Relationships can enhance the schema's functionality by connecting various components or datasets without altering the locked fields themselves. This means you can still expand the schema's capabilities and enhance data interconnectivity without directly modifying the immutable fields. Thus, the ability to add relationships provides flexibility and keeps the schema robust while adhering to the constraints of immutability for locked fields.