/z/ → /ʒ or ʃ/
The voiced fricative alveolar /z/ becomes a voiced fricative post-alveolar /ʒ/ when followed by /j/. In theory, this should happen in the same way when the next sound is the voiceless post-alveolar /ʃ/, and this is what most books about English pronunciation say. However, in practice, that sound is devoiced by the proximity of the voiceless /ʃ/ -remember what we said about the devoicing of final consonants-, so it turns into a /ʃ/ as well. The sequence /z/ + /ʃ/, then, is very often pronounced /ʃ ʃ/, which, unlike the cases we have studied so far, also implies a change in voicing .
/z/ (followed by /j/) becomes /ʒ/
As we saw in the previous article, sometimes the sound /j/ can still be heard, so the resulting sequence would be /ʒ j/. In other cases, however, the /j/ is not heard because it has been completely assimilated into the new sound.
/z/ (followed by /ʃ/) becomes /ʃ/
And here are some examples:
In this recording you can see the assimilation case where there is a change of both place of articulation and voicing (/z/ becoming /ʃ/):