I have multiple ecobee thermostats at my home. Do I need to replace all my thermostats if I choose to buy ecobee3 lite?

No. Your ecobee3 lite will work alongside your other ecobee thermostats. When you register your new ecobee3 lite to your account, you will be able to access the new web portal, which is where you’ll be able to control and manage all your thermostats.