I had a similar problem with UR and a UA ticket - UA cancelled a leg - and UR said to contact UA, UA said only UR could fix it. Eventually a UR agent checked with UA whilst I waited on the line, and got "permission" to change the reservation. It did take nearly an hour on the phone!

HUCA is what I would recommend if they say they can't help - suggest they call the airline (they should have different phone numbers). Or ask TK to put a message in the PNR asking UR to change it (I did that once).