Unless you insulate the piping, 3/4" copper has a much higher ratio of volume to surface area, plus copper is a better heat transferer, so you will get more heat loss with the baseboard. You would probably have to break it down into multiple zones so that 3/4" would be adequate. Other than heat loss from the piping, you are only heating whatever it takes to replace the heat used to warm the rooms. Baseboard may, or may, not heat them more efficiently and if so that might generate some savings. Whether it would be enough to compensate for the cost of the conversion is debateable.