No. All of our thermostat models are compatible with each other: If you already own an ecobee4 and are looking to purchase a new model, e.g. SmartThermostat with voice control, you won't need to replace your ecobee4 to do so. The same goes for upgrading from an ecobee3 lite to SmartThermostat with voice control. If you're looking to add additional thermostats to your home, you can mix-and-match different models knowing that the thermostats will work alongside each other.
When you register your new ecobee to your account, you'll be able to access it in the mobile app and through the Web Portal. From there, you'll be able to control and manage all of the thermostats linked to your account. You can add up to 16 thermostats to a single account.