Disco 1 Changing turbo cassette query

Hi All. Need some advice. I need to change the cassette on a disco 200 tdi engine fitted to my defender 90. There looks to be lots of difficulties in removing the whole turbo, especially with the studs/nuts holding the turbo to the manifold. I wondered if anybody knows of a reason why a turbo cassette could not be changed with the turbo housing remaining in situ?