Carry on using them, reset and gift them to a friend, family or charity - or return to Sonos for recycling.  They carry on working after you have taken advantage of the upgrade program.

Factory reset before recycling. This will remove your details. Note that, unless a new user adds the Reset unit to a system, the unit(s) will remain part of your profile — even if the unit(s) are no longer functional.

Factory reset.

Set up again using a throw away e-mail address and account. (clearing your real account)

Reset again and recycle.