If you've ‘registered’ them to your account (and not added someone else's to your system), they should be showing up as registered to you at www.sonos.com. I’d recommend that you call Sonos Support directly to discuss it. I suspect they can look them up and see why they’re not showing up for you. 

I bought them second hand, so I imagine they were previously on someone else’s system. Hmm I guess these CR200s are different than other sonos products then, because everything else if I bought second hand would automatically show up for me on sonos.com. Do you think they can somehow register these CR200s for me?

Note that a given unit can only be used once for the discount, regardless of the current owner.

In order to show up on www.sonos.com, they’d need to have been factory reset (either by the previous owner, or by you) before ‘adding’ them to your system. With, of course, the caveat mentioned by ​@buzz, there can be only one upgrade consumed per device, no matter how many owners. 

And frankly, given that the CR200 stopped being supported long ago by S1, I wouldn’t actually know how to either factory reset it, or add it to a ‘new’ system. I think the devices were unsupported back in Sonos S1 OS 8.3, but that’s a guess. Your best bet, both to see if they can be factory reset, and to see if their upgrade hasn’t been already used, is to call Sonos Support directly to discuss it.