You do not need to send the item back.  You can if you wish to have Sonos recycle the device, but it is not required. 

Thank you for the clarification! My old device still works fine, just wanted to get the new Arc and try it :)

Seems like anything to do with Sonos is complicated, even this upgrade thing. How many folks will send back a working speaker because they got an upgrade? Only Sonos  knows for sure, but my guess is 2% of the people smart enough to figure out how to use this "credit", will sent the old one back. How does someone get the credit for an upgrade? 

Did you even read the thread?  As stated, there is no requirement to send the speaker back after the upgrade is used.  You can recycle it, you can sell/give it to someone, or you can keep on using it yourself.  

As for getting the credit, go here https://www.sonos.com/en-us/upgrade scroll to “How It Works” and follow the directions.  But a warning, you might have to make a very difficult click on an “Upgrade” button, so be prepared for the strain. 

 So the question was, what % of Sonos users will send there speakers back. Only Sonos can say, but I stand by the 2% or lower guess. In fact, I wouldn't be surprised if no one sent anything back, but it's a good thing there is that tab centrally place to get the UPS labels, just in case one or two people do decide to send something back. 

Now excuse me, I'm troubleshooting the Sonos Play 1 and Move that are not showing up in the app, although the Move is currently playing music. Had to power cycle the router, and now will once again see if these two speakers will show up in the app. Complexity ha, this just plain fun figuring out network issues right before the party. 

There is no additional percentage given to those who send back in their speakers, it entirely voluntary, not rewarded. 

One thing I will say is be sure to check that the offer is worth it. Recently some of the deals that are available from other retailers have bigger discounts. The main Sonos store has everything at full price. EG. Sonos Move in UK can be had for £299. It is £399 on Sonos store if you log in to use your trade in allowance. 

The original upgrade scheme would brick the trade-in units via software. This created a public relations mess and the scheme was changed to the current arrangement where SONOS only allows each unit to trade-in once, but remain operational. You may have come across some stale content or discussions on the Internet referencing the original scheme. The current risk is purchasing a “used” unit, expecting to use it as a trade-in, only to discover that it has already been used as a trade-in.