There aren’t many restrictions in terms of what you can use the upgrade discount on. One is that the Series 1 Bridge can only be used as a discount against the Boost, and generally older products that can work in both S1 and S2 systems will only attract a 15% discount, rather than the full 30% for S1-only products.

A summary of the terms is here:

https://www.sonos.com/en-us/upgrade

The complete conditions, including all exclusions, are here:

https://www.sonos.com/en-us/legal/upgrade-terms

Somewhat helpful, but I still can’t figure -- if I am replacing a system that has 5 separate components, not including the bridge, but only want 3 new components, how are these discounts applied? 15% off what? 30% off what? 

Sorry, but you just need to read the terms - specifically the ones in the second link.

The Upgrade Terms are completely explicit - they explain unambiguously how the discount from each upgraded item can be used.

Thanks, you're right of course. It’s always intimidating digging into these kinds of documents. I should have looked.