Upgrading a multi-zone AHU

[Norm01]

I am working with a client who has an exisiting multi-zone air handling system (approx. 33 yrs old) with DX cooling and hot water heating. The building is two story ofice buidling with approx. 15 zones (there doesn't seem to be any purpose to the choice of zones and with renovations over the years you have one thermostat serving both exterior and interior zones). All controls are pneumatic and individual pneumatic actuators provide supply air to each zone. They have added reheat coils to some areas over the years to improve comfort. They are looking at upgrading the system to be more efficient and to make employees more comfortable.
I am wondering if this type of system is taylormade to be upgraded to a VAV system (with VFD's on the fan units)? Any other thoughts on upgrading older muli-zone HVAC systems?
Thanks.

 

[remp]

Does the AHU have a hugh plenum at the outlet with 15nr. different branches from it? does each branch have a Heating coil and DX coil, or is there just one dx and one HC at the AHU upstream of the duct splitting into zones? Is there a reheat further down the duct in the rooms???

 

[Norm01]

The AHU has one DX coil and one HC and then it goes to a plenum and braches out to each zone separately. There are some reheat coils that have been added with office renovations.