No. All of our thermostat models are compatible with each other, so if you already own an ecobee3 and are looking to purchase an ecobee3 lite or an ecobee4, you will not need to replace your ecobee3 to do so. The same goes for upgrading from an ecobee3 lite to an ecobee4; if you're looking to add additional thermostats to your home, you're able to mix-and-match different models and the thermostats will work alongside each other.
When you register your new ecobee to your account, you will 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.