To the best of my knowledge, you cant just revoke credit cause you feel like it. It would lead to a lot of perverse incentives (revoking fails, etc).
Two main situations, one as SBC detailed with transferring, another is in case you are seriously ill or impaired only after the census date and thus had to finish the unit anyway. In this case it may also be revoked.
In terms of messing up your subject selection (hey, we've all been there), you'll probably have to lie in the bed you've made and just do the best with what you've got (again, i've been there).
If it makes it impossible for you to graduate, you may be allowed to do other units on a normal basis but i've also seen people offered to do them on a full fee, single unit basis as well.
As with all things, contact the faculty to be sure.