There are two types of upgrades, and it’s not clear which you’re asking about.

First is the upgrade from S1 to S2, which I did on my PLAYBAR, too.
 

The second upgrade is ‘trading’ in the PLAYBAR for a discount on another Sonos device. I did this for an Arc, so it was in my mind an upgrade, but the way Sonos has made that system, you can use that discount you receive and apply it to any order, not just the ‘upgrade’ model. The side benefit is you get to get the discount, and keep the old device and continue to use it, as you choose. I ended up gifting my S2 PLAYBAR to my sister. However, that discount per eligible speaker is available once, so since I’ve used it, my sister will not be able to do so.