I already have many ecobee residential thermostats in my businesses. Can I use these with ecobee SmartBuildings?

Yes.

If you already have ecobee thermostats, all you need to do is purchase subscriptions and remove the thermostats from any other accounts they are registered to. Unregistering the thermostats will not affect the settings of the thermostats and they can be simply re-registered into SmartBuildings.