I thought that the requested SDC flight can be in any fare class? Not just the original booking class? I booked it in E (and there are no more E seats left), shouldnt I be able to take any U,V,K,W, etc seats within 24 hours of departure?

No. It must be the exact fare class to SDC. You can standby but not confirm if the specific fare bucket isn't available. The good news is that they generally open them all up ~3 hours prior to the flight so if it is Y9 now and they don't sell the seats you'll get to be E9 eventually and then you can confirm.

I was able to call in to the Elite line for an SDC for my United codeshare BWI-ORD yesterday. The agent offered my either another United direct flight or a CO flight through either CLE or EWR. I chose CO6469 and am glad I did, even though it left an hour late, my orginal CO6765 was first delayed for an hour and then cancelled at 10:00pm! I arrived in ORD at a nice 7 pM!

United's check-in kiosk and Counter agent were not as user friendly. First they tried to charge us for all 2nd piece of luggage, and the agent said she would take care of it through her. When two peices came in at 53# and 61# she wanted and extra $100 per overweight bag. I told her I thought that as (3)*G's on CO tickets originating on CO metal we were entitled to 3-70# bags apeice. She told me that no, as we are now flying on united for this leg, we are only allowed 1-50# a peice. I re-sorted some items, and got down to only one of the 4 bags being overweight, and was resigned to paying up the $100 and fighting it latter. She looked disapointed when she imput all the needed bits of info in her terminal, and no charge appeared to process.

I asked her if there was any chance of us getting up into E+ as *G's and she said yes, for $29 apeice! As we were already sitting together vertically, (22E, 23E, 24E) I almost bit at the offer, but I thought I would give the GA a chance to make up for a crappy Counter Agent before shelling out the $$$.

When I asked the GA if there was any room in E+ for a CO *G she said she wasn't sure, and that she would have to look up the rules. I explained that the system was not in place yet and that some had reported good luck with nice GA's. She first said that maybe for one pax she could but not for all three, when I pointed out that all three were CO*G's she gave us 7A,B&C!!!^

The agent was wrong. As a *G on UA you get 3x70#. Was it a CO or UA agent who gave you the bad info? I'm guessing UA since you'd be checking in with them if flying them.

Your logic is spot on! It was a UA agent.

All you need to remind them of is *G. You should have had the higher baggage allowance.

I'm surprised this was the case. I read on CO's website that SDC was only available for CO metal flights. Last week I flew SFO-PHL on a CO codeshare (UA metal) and fortunately the UA ticket agent let me SDC without the UA $50 fee.

There seems to be a bug in the CO implementation where it is permissible on any CO-coded flights, not just CO-code AND metal flights. Or maybe they did it on purpose, but it seems like it is a bug given what they stated as policy.

So it CAN be done the website just needs to be updated?

I misread your post before; I'm not sure why the UA agent waived the fee.

But, yes, based on empirical evidence shared here on FlyerTalk it seems that a CO-initiated SDC can switch to any CO-coded flights, not just CO-metal.