Nick, as you know you can only "grandfather" when you don't change the design. The EC225 has all-new blades so for your information they are of course compliant with the latest version of JAR and FAR29 for lightning strike tolerance.
Nick-bashing aside, I would also point out that whilst the manufacturers are well aware of the maximum action integral of lightning that their blades must tolerate, Mother Nature is not so well aware and may decide to discharge more than the maximum into your blades - so best to avoid getting struck!
On the metal versus composite blades argument, neither likes lightning very much. The metal blades tend to have pockets blown partially off which really upsets the aerodynamics causing very severe vibration. And as we know there is the possibility of damage to the spar. The composite blades tend to vapourise on the inside blowing off big chunks and leaving steams of fibre trailing. But overall I would prefer fullly composite blades because they don't have spars - therefore they tend to be more damage tolerant (distributed loads and all that). I have seen some scary-looking blades that flew for 100miles or so with the pilots blissfully unaware.
Apart from the blade damage the whole drivetrain may need replacing due to magnetisation and arc damage - a very expensive excercise!
In the North Sea we (as a group) get struck by lighting typically once per year, nearly always in winter and nearly always with the OAT around zero. We don't normally get struck by the sort of lightning that comes out a big red (on the radar) Cb, instead it seems that the aircraft triggers the lighting by its passage. Probably the trail of ionised air that is generated by static discharge and engine exhaust gas - a bit like trailing a long wire behind the aircraft.
And its no co-incidence that it occurs typically around zero C - at this temperature there is mixed-phase water (ie liquid and solid water). The solid is lighter and tends to be pulled up in the updraft, whilst the liquid is heavier and tends to sink. This pulling apart leads to charge separation with the maximum electric field strength near the zero degree level.
And that explains why we only get struck in winter - because only in winter are we flying around near the zero degree level.
Or something like that.....
HC