The whole concept of "switching" between SC and SN seems cumbersome.  I
haven't really thought through the technical issues, so this may not be
feasible, but here's what I think the user experience should be like:

When setting up a new player, the user should have the option of
enabling SC, SN or both.  The default should be both at which point the
player is essentially connected to both SC and SN.  Exactly how this is
done and which side does the heavy lifting isn't clear to me...
Ideally, the exact same webUI should be available both at a local url
(SC) and an sn.com URL.  Any options, prefs, favs, etc. would need to be
saved on both SC and SN.

If the connection to SC is lost (perhaps the PC was turned off), the
only change the user will see is:
- The webUI is only available via the sn.com url
- The playerUI and SBCUI are unchanged except that "Music Library" is
unavailable.

Similarly, if the connection to SN is lost:
- The webUI is only available via the local URL
- The playerUI and SBCUI are unchanged except that music services
requiring SN are unavailable.

This could probably be extended to also allow simultaneous connection to
multiple SCs with multiple libraries.