Being Scots and, er, careful with money, I would check the shocks to make sure the damping is still OK and there are no leaks and then re-use them. Original shrouded Girlings were well made and do last a long time. Whereas double-damped shocks may well be the bee's knees, do they make any discernable difference to the normal rider? I have so far clocked up 50 years riding on Girling and latterly Hagon shocks - even racing on them moderately successfully - and they certainly did all that I asked of them. As for frequent changes of shocks - every 3rd chain would equate to every 18,000 miles for me - seems a bit over the top. Can I have the old ones please?