If there is a posted delay to your confirmed flights (i.e. on the flight status -- to either DTW-IAD or IAD-FRA! probably IAD-FRA would require a significant delay to be considered inconveniencing), you would be able to suggest an alternate itinerary. I would recommend setting a flight notification ("Easy Update flight notification" via the UA home page) to keep you somewhat in the loop regarding delays, if you do not have other means of finding flight info on your day of travel.

If it is known early enough in the day, they would probably suggest routing you via ORD (see the schedule links provided in post #2 above), but you could also ask for the 4pm LH nonstop if you desire. If the delay is not apparent until close to your 7pm flight time (and keep eyes open to watch the weather and if similar flights' departures are being delayed out of DTW), then it might be advisable to tell the GA you cannot get on that flight because it is a guaranteed misconnect in IAD, and can they rebook you on one of the remaining options. The tricky thing is if you board the flight and it's then stuck on the ground with a long delay -- then you have to see if it is a significant enough delay to ask to get off. But that's sort of an advanced topic that others can cover if desired.

As for who helps you do this, possibly the phone reservation agents could rebook you over to the LH flight (and certainly give it a try if you have not left for the airport yet), but they only go by the official flight status to see if there is a delay, and I would say the airport agents are more likely to be able to assist you if it is becoming apparent that it will be delayed, but not posted officially yet (if you're already at the airport from the noon flight you're trying to go on). Also they will need to take care of your bags following you.

I would approach it by asking the relevant GA or checkin agent for help, saying that your trip depends on getting there today, due to flight connections, meetings or whatever reason, and there is an alternate routing with the LH flight that they can access as UA codeshare inventory (so easy to search on their computer), which puts you in FRA at nearly the same time as you planned. The 10pm LH flight has at least several seats available, and I don't imagine they are going to fill up between now and then (and there is plenty of non-codeshare inventory on the LH side too, see here). If you're put on the LH codeshare, you will not pay anything -- it's UA's responsibility to rebook you.

hope it goes well for you, and would be glad to hear about how it went if you update this thread with the result in the future!