The boundaries of relevance

What are the boundaries of relevance with G? For example, if I decide to change the niche of my site a little bit, how far is too far in respect to the current PR, backlink relevance, content relevance etc?

How closely related do niches/topics/content need to be in or to be considered relevant by G?

In my opinion people give waaaayyyy too much weight to the "relevance" idea.

Sure getting links from relevant sites is great to drive targeted traffic, but from an SEO standpoint I always go with power of the link over relevance (sure it's great if you can get both).

I manage a lot of sites in pretty competitive niches, and I see a lot of them use paid links from PR7-PR8 sites unrelated to their niche with much success. But so I digress.

Anyway getting back to your question, a slight change shouldn't affect you that much, unless you are changing the content you are currently ranking for pretty drastically.