i think retro lids may not be the best idea ever, read this quote from MET;
"We are often asked 'For how long is a helmet safe?', or 'how often should I replace my helmet?”' Until now it has been difficult to find any reliable figures to help answer these queries. MET have now developed a series of tests which are conducted on aged helmets to determine a 'best before' date (unless the helmet is involved in an accident. In that case it should be replaced immediately.). The results indicate that, if used properly accordingly to our owner manual, our helmets will still do their job up to eight years after they have been made. Not only is that good news for the customer, it’s great news for the environment!"
it would be horrible if some one got hurt for no reason, just my view though...do what ever you want..
I've always brought a replacement helmet every four years, as that was what motorbike helmet makers recommended and they are more or less made out of the same stuff. I've never found it easy to get rid of old helmets though, maybe one day I'll turn one into a cool hanging basket.