So you replaced an Xbox with a new one and now you can't get the google assistant to work?
1. Logon to your New Xbox and then go to settings > Devices & Streaming > Digital assistants > Enable Digital assistants.
2. Now, set up a new device in Google home on your phone (link your Xbox service)
3. Profit.
How to remove the old device?
Turn your old Xbox back on, disable the digital assistant instead of enabling it.
What do I do if I don't have my old Xbox?
NFI? Pray to Microsoft? Call for support?