'66 Sprint GT, '67 Duetto, '70 BMW 2800CS
You have been told the truth!I have been told that the top chain is possible to replace without taking off the cylinder head however the bottom one does require the head being removed.
As alfar7 says, you can install a new upper chain without removing the head. I use a length of coat hanger wire, with a hook formed at one end, to fish the chain around the intermediate sprocket.
However, to replace the lower chain you'll need to remove more than the head. The front cover needs to come off, and to do that, both the head and oil pan need to come off. A lot of work if the engine is still installed in the car.
Why do you feel the chains need to be replaced? Usually this gets done as part of an engine rebuild.