To change the username that your ecobee4 is registered to you must first unregister your account, and re-register under a different email address.
It’s important to know that when you unregister your device you will lose your previous HomeIQ data.
To un-register, go to your web portal and select the drop-down list at the top right. Select MY ACCOUNT. Then select UNREGISTER (if you have more than one ecobee thermostat on your account, make the sure the one you wish to unregister is selected). Select UNREGISTER at the bottom right once you ready to remove the ecobee4 from your account.