Replacing Chain Guides

[thom4782]

My 'left you stranded' post leads me to think more about preventative maintenance. Specifically, I'm wondering about the chain guides.

As I understand it, there are two types in my car: variocam rails which are brown and chain tensioner rails, which are black. I have a couple of questions.

1. In the absence of any symptoms, like plastic particles in the oil filter, should one replace these items proactively?
2. If so, at what mileage should one consider doing the work?
3. Can one replace the chain tensioner rails without splitting the case?

I've looked and haven't found a post on point. Any guidance would be appreciated.

Thanks