Both Amazon Alexa and Google Assistant use the same Amazon account to recognize your devices.
To switch your account you must log out of both Amazon Alexa and Google Assistant in the iDevices Connected app. Then log in with your new account in the iDevices Connected app.
For Google Assistant, you will need to log in with the new Amazon in the Google Assistant app as well.
To switch your Amazon Alexa and Google Home in the iDevices Connected app:
1. Tap the 3 lines in the upper right hand corner (☰)
2. Scroll down and tap "Settings"
3. Select "Amazon Alexa"
4. Select your account
5. Select "Log Out"
6. If you're using the Google Assistant, select "Google Assistant" and repeat steps 4 & 5.
7. Sign now select "Amazon Alexa" and "Google Assistant" to sign in with the new account.
If you're using the Google Assistant, you'll need to link the Google Assistant to the new account:
1. Open the Google Home app
2. Tap the 3 lines in the upper left-hand corner (☰)
3. Tap "Home Control"
4. Tap the 3 dots in the upper right hand corner and select "Manage accounts"
5. Tap "iDevices" and select "Unlink Account"
6. Choose "iDevices" from the list and sign in with the new Amazon Account