I agree with jsloan - everything I see suggests that you should be able to change to a LON-CHI fare on the return using historical fares and AP without issue.

That being said, I've had an abysmal record with getting UA to do things that make sense with complicated faring; once I was changing to a new ticket (current fares) and it took a transfer to Web Support to show them the fresh itinerary as bookable on .com before they agreed the price was right.