We are in the process of changing our small steam heated library to hot water heating on an extremely limited budget. Three of the existing zones are not presently being used but our heating engineer suggests changing out the existing pneumatic actuated steam valves to pneumatic actuated hot water valves as there is some difference between a steam valve and a hot water valve. In view of the fact that we will probably never use any of the three (two cabinet heaters at front and back door and one overhead heater in a cold storage area) is their any real need to change out these valves?

If you are not going to use them cap them off or disconnect them from the loop. Changing from steam to water is not as simple as it seems. I suggest you mention this to the heating engineer and see if he feels that will work.