جَاءَ أَمْرُ رَبِّكَ ← وَجَاءَ رَبُّكَ
The underlying wording “the command of your Lord comes” is reduced to a form in which rabbuka has a different case.
In the first construction, “amru” is the nominative subject, while “rabbi” is the genitive complement. In the expressed wording, “rabbu” is nominative because it occupies the subject position after omission of the possessed head. The second-person suffix remains attached to “rabb” in each construction.
The relevant change concerns the case assigned to “rabb,” not a change in its letters or a claim that its lexical identity has disappeared. This is the reason the example belongs under change of case through omission.