What makes an engine 1.5 or 1.6 - HEAD?

I've got a 1982 1.5 GL cabrio (97k on clock) but the engine code on the block says its an FR engine which I believe to be a Scirocco GT 1.6 carb engine. Problem is the head appears to be the standard 1.5  head with pierburg carb and manifold. Would this still be classed as a 1.5 engine? Will running this head do any damage to the engine- compression etc?

Its started puffing blue smoke under revs so I've got hold of an FR head and manifold with a weber carb on it. Problem is I can't afford garage bill to have it put on. How big of a job is it to swap a head?

Possibly they lengthened the stroke, they do this with the JB engine too.