Here's the conversion chart...
http://www.games-workshop.com/gws/content/article.jsp?catId=cat730002a&categoryId=6800002a§ion=&aId=21600002a
So the replacement for
SW grey is Fenrisian Grey. I couldn't tell you how close they are, I haven't bought any Fen. Grey. Some paints are very close matches and others aren't close at all. You'll have to find someone who has tested it and can compare, and even then some people don't notice the differences as much as others. One sure fire way is to test it yourself

My local
GW is quite happy for me to go in and grab a swipe of the new paints on a model to compare it to the old ones.