In order for a basic income program to save costs on bureaucracy and replace existing programs, those existing programs must be closed by legislation or executive order, and the people employed by them must lose their jobs, and the number of employees in the new basic income agency must be fewer. Many of these employees will not find replacement work due to age or their industry (government bureaucracy) shrinking so will go from middle class to poor on basic income.
This will have a large economic effect unless it is done slowly, but if it is done slowly, both programs will have to exist at once without paying too much to the same person. This will require the basic income agency to have the same kind of bureaucracy as the other agencies or else have no oversight. In the first phase of the transition, at least, it would cost much more.
The agency would need to maintain some bureaucracy to guard against fraud since obtaining benefits would never require a visit to an office or proof of some activity. It would be easy for someone to claim extra people without oversight.
This will have a large economic effect unless it is done slowly, but if it is done slowly, both programs will have to exist at once without paying too much to the same person. This will require the basic income agency to have the same kind of bureaucracy as the other agencies or else have no oversight. In the first phase of the transition, at least, it would cost much more.
The agency would need to maintain some bureaucracy to guard against fraud since obtaining benefits would never require a visit to an office or proof of some activity. It would be easy for someone to claim extra people without oversight.