How can I upgrade my bridge if I can’t connect it to my S2 system
The Bridge is not compatible with S2. You need to replace it with a Boost or wire one of your speakers to your router.
I think the question may be that the OP simply wants to connect it temporarily so that he can get the discount eligibility to upgrade it to a Boost - but the question is a bit too vague to be sure.
If my guess is right, it should be possible to simply install the S1 app on any phone/tablet that will run it, and then factory reset and install the Bridge as the only device. This will allow the Bridge to be registered on your account, and for the upgrade eligibility to show. You can then delete the S1 app from your phone/tablet.
Mod Edit: Changed “Boost” to “Bridge” prior to marking as Best Answer.
I’d just try calling sales with the Bridge handy to read the serial number off, might save messing around.
The device should show up here https://www.sonos.com/myaccount/system/households/devices/
with a 30% discount option for upgrading the S1 device.
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