@Slingshot75 

You can do one of two things.  

Option 1:  swap the location of the two units.  

Option 2:  Log into the Deco app Tap on the network map and then the unit that is downstairs.  Finally mark it the main unit.

After doing one of the two you will just need to worry about the WAN/Internet being configured for the new ISP's service.  Most ISPs in the US are now dynamic so that shouldn't be a big concern.  I would say that if your ISP is providing you the modem, make sure they do not give you a gateway.  This is a modem/router combo device and would create a double NAT situation with the Deco.  You want to avoid that if possible.  If they say you have to use a Gateway, like when using Verizon Fios and having Fios TV service too, then you may be required to put the Deco in AP mode.  Note doing this will disable all of the router functions of the Deco and just regulate you to the Wireless LAN features.

  @Slingshot75 

That is what I am using right now.  As long as your ONT is not a multi-port OLT you should be fine.  The Multi-port OLTs are generally Gateways and those could create concerns like Double NAT.  If you are just getting an ONT then you should be good to go.