To circle back on this, after an unhelpful UA phone agent, and no time to HUACA, I just went to the airport early. There is no ticketing counter/office at Munich anymore; TSA contractors didn't know where I should go, I managed to catch one of the check-in staff who told me they can do it all on the check-in counters, just to check in with TSA first. As my routing was MUC-LHR-DEN (so, not originating on United from MUC), the TSA contractor wouldn't let me pass, but took my passport to a UA supervisor to ask... Then she returned with a laminated sheet with United's German and US reservation numbers and said they can't do anything at the counters, and I should rebook myself over the phone... 🤦🏻‍♂️

The phone agent denied SDC, since first flight is operated by LH (though on UA codeshare flight number on UA ticket stock...). It's funny, because without that first LH leg, there would actually be more money left for UA on a more direct UA-operated routing, but hey...

My intention was to get myself rebooked, then upgrade to business at online check-in. Since Polaris is flying literally half empty on ORD flight today, I was guessing I could get a decent price for it. Alas, it wasn't meant to be.