Your old stuff will eventually wear out. I'd just replace it when it does. There will come a time, like when a wheel goes, when it'll be less expensive and less hassle to go to 8s rather than stay vintage. I'd wait for that, then spread the stays and just start replacing as I need. That'll be the big jump, when you replace the hub. Your old Ders and shifters should be fine with 8, just not indexed.