Every other Varna in your post seems to be changing depending on the economy and demand..for eg Kshatriya to Shudra..Shudra to Kshatriya..Vaishya to Shudra and vice versa.
But why no mention of Brahmana Varna?
Has this Varna changed into any other Varna becos of economic demand reasons?
No. Kshatriya to Sudra varna (i am not talking of people or jAtis here) differentiation is so thin. Service provider can become laborer (if skill becomes obsolete) if service is not in demand. If skill becomes in demand then laborer (sudra) can become Kshatriya (service provider).
Material providers (Vaishyas) and Knowledge providers (Brahmanas) nature is not that.
This is not about jAtis, this is not about people migrating from one profession to another. This is about the nature of the varna. At all times, these varnas will exist and this is how they will exist. That is dharma of the Varna.
Today all people in Brahmin jAti are not Brahmanas (knowledge providers). Some are Kshatriyas (service providers like doctors, self-employeed engineers eetc), some are zudras (unskilled/getting obsolete skilled labor like clerks, peons, jobs that would be lost in course of time), some are vaishyas (farmers, landlords, traders, businessmen).