The 797 isn't even official yet and too far into the future. UA is in a pickle with the 757-222 and 763 fleets. I expect that the NGS upgrade will happen. As for the speculation about Boeing restarting passenger 763 production it sounds unlikely. United also won't be able to source 35 newer 763s on the secondhand market to bridge the gap until the 797 is flying. Perhaps they'll be forced to take more 787s or literarally fly the wings off the old birds.