Theoretically he is correct.

Old heating system can leak, radiators and old pipework can leak, ballvalves currently on low pressure should be changed to high pressure, existing taps, showers etc will be low pressure so can drip/leak and need replacing, as may radiator valves.

Modern boilers may be difficult to locate according to the regs about flue positions etc.

And then you have the biggest problem of all, the cold mains.

You need to think very carefully, and get the mains tested properly at peak and off peak times for flow rate (Ltrs/min) and pressure.

As a very minimum you would need a 32mm supply and possibly 50mm.

As above Theoretically he is correct, but that depends greatly on the age of the existing, I would say some leaks were inevitable on an older system.

Damage limitation can be achieved, by pressure testing the existing pipework, during the change over.

What would normally happen, is you would have a service room, with say a 50mm supply, the 12 stopcocks and 22mm feeding each unit.

The gas would be the same, 12 meters, one for each unit, from a central area.

Personally I don't think it's the way to go, but that depends on the occupancy etc. are they let, lease or owner occupied.

District heating and hot water systems have many advantages, slashes your carbon footprint, can be used with renewable energy, solar etc, and invariably a lot less problematic to install in an occupied building.

You should look at the options, and take the best system within budget, not forgetting the access problems, unless you intend to chuck them all out for 3-4months.