I was thinking about doing this with my '80 928, and understand that once the material that makes up the vacuum lines gets old and brittle it becomes an on-going problem due to leaks and loose fit. Someone advised me that in addition to the vacuum hose, there are a lot of vacuum accuators that also get old and brittle. And that same person suggested that changing these accuators is a good "while you're there" opportunity. And a lot of electrical contacts that might want to be examined and cleaned, while you're there….. So it becomes a big job. Worse is that that in pulling everything apart there's a better than even chance of introducing new issues.
Even though in the last 2 years my car has had about 4 hoses (water), 2 fuel related (injector system), 2 vacuum (engine area), and 1 (hydraulic) replaced due to age, I'm still debating. Let me know how it goes!