Forgive the tangent here, but in my opinion, the "cross-compiling" to x86_64-unknown-linux-musl in multirust is really really good. I'm not sure how it could be easier.
I like the simplicity of this mode of operation, but it doesn't feel like it's the exact right thing to do. I think my preference would be to have a bare "rustup" just print usage (just like multirust, cargo, and rustc). I don't upgrade toolchains often enough for me to get the benefit of not having to type a few extra keystrokes.
But as designed, both rustup and rustup update seem to work well as described.
nightly updated - rustc 1.9.0-nightly (b678600ac 2016-03-29)
In this message I wonder if you can print the previous version I had, so I can revert if necessary.