If you can get hold of the UA PNR--record locator (not the one issued by AE)--then you might be able to do a flight change at IAH by using one of the touchscreens they have at the flight assistance desks (I don't recall what they're called precisely now, but you'll see them). Typically for me, on UA flights, once under way I've been able to make changes at the airport that way. There will likely be a fee, however, for SDC. They also have phones that are directly linked to phone agents. I've found them to be very helpful when I've had IROPS and/or needed other changes. They should be able to pull up the UA locator for you and advise you on what can be done.

IAH is not a great airport for the kind of thing you want to do anyhow as the airport is a bit of a trek into the city. Houston does have some lovely museums, which are worth a visit, but I wouldn't consider doing a 12-hour layover for that purpose.