You have no clue what you're talking about. You are correct in that low cost PLC hardware can replace relay and pneumatic controls, but HVAC systems aren't just basic relays and pneumatics. They are microcomputers that communicate with actual protocols. They is no hot cutover or simple handover like you say.

Source: I literally just lead a project to upgrade the PLC controls on a few air handler units and build entire server and network infrastructure. We had to run Ethernet to each of these from the network closets because they previously had went to a Windows XP box in the maintenance guys office. There were many challenges, one being getting the configuration off a couple of the old PLCs to make the new ones even talk to them.