A couple  different questions here.

1 - Can you switch the outdoor speaker to run off your downstairs amp instead of upstairs?  It depends on what amps and speakers you have as well as whether you can physical rewire your speakers to a different location.  The better question is, why do you want to do this? Does your upstairs amp also power upstairs speakers, and you the outside and upstairs to play different audio?

2 - Can you have your Arc play different audio than what’s playing on your other speakers and amps?  Yes.

It might be helpful if you stated what amps you have, are they Sonos Amp?  Sonos Connect:amp?  Some other brand of amp?  And how many speakers you have connected to each amp? Where are the speakers located?  A lot depends on those answers.

1. Not as many people use the full surround speakers downstairs.  I have my ARC downstairs and just watching regular TV it does great.  My upstairs SONOS AMP is already hook up to my living room and bathroom.  More people are upstairs and can be listening to something different than what someone is listening to outside.  I can also rewire my speaker to go to a different amp.
2. I have the SONOS AMP.  Do I need to have my arc not connected to my amp.  I have a switch so that I can turn my surround to the TV or listen to music.
3.  

Ok, so you would need to reconfigure the downstairs amp as it’s own room in the Sonos app, connect your outdoor speakers to that instead of the surround speakers.  Once you’ve done that, you no longer have surround sound, obviously, but you will be able to play music on the outside without out having it play anywhere inside, at whatever volume you want.

You have two Sonos amps, right?  I really am confused as to what your setup looks like, as I’m not following how you have a switch (speaker switch?) downstairs that you use with surround speakers and...some other speakers?  

Yes I have 2 sonos amps.  I was told that I could for my downstairs have a switch and the rear speaker can be music speakers or surround.

I can’t do anything with that but ask more questions.  Told by who?  A speaker switch?  If your rear speakers are powered by a Sonos Amp and used as surrounds with an Arc, there is no need for a speaker switch.  The speakers will already play surround or music as you wish.  You would only use a speaker switch if you want your downstairs amp to connect to multiple sets of speakers.

I was told by my Sonos dealer.

My downstairs has a Sonos Amp, Arc and to rear speakers.  I have a switch so that I can switch to music, using my app, on my rear speaker and still use the ARC for the TV