Thank you all for pouring your expertise in each case. I've been reading you religiously.
I've been hit! repeat, I've been hit! And hard. Originally it was Austin to London via Houston in late December. Now I'll have to depart 15 min earlier, stop in Chicago and arrive 30 min later. Flight numbers changed. Even my seat numbers changed (and I paid for Premium Economy!).
I decided to accept this change as my research shows ORD transfer should be doable and I have flexibility for earlier departure and late arrival. C gates to B gates in worst case scenario, walking through the Neon Light Tunnel. Hope weather is on my side as it would be mid/late December.
United app shows I can do a free one-time change or a refund. I'll save that option for now just in case.