You would have to replace everything, probably including the actual heating/cooling systems themselves.

You wouldn’t need to replace the actual boilers/chillers/air handlers/terminal units, just the temperature controls/control wiring/control relays/control computer. There is mechanical equipment out there that is still controlled by pneumatic controls and that equipment can be retrofit to use digital controls.

Replacing the mechanical units may be beneficial, condensing boilers are extremely efficient and so are newer chillers/etc.