Can old TH115 be switched out for nSpiration Series without any alterations to the system?
Switch out should be very quick and easy process, since the mounting is about the same. Electrician would just simply need to dismount old unit and disconnect all the wires (supply, load and floor sensor). nSpiration Series Thermostats have clearly labeled and easy to use screw-terminals on the back for supply and load. Floor sensor need to be connected to the front of the base, just like in the previous model.
This document should have all the information regarding wiring.