I'm not sure they are "waiting" for anything. When they announced that huge 737-900ER replacement order last summer/fall, didn't the company state then that these deliveries would be for the second half of 2013? So if the -900ER is replacing the older UAL 752s and they are expected to start arriving after June 2013, it then follows this is when the 752s will start exiting the fleet. At least that is the way I read the original press announcement. This really should come as no surprise.

As a side note, I laughed when I saw the title of this thread. Based on what I've seen so far "UAL" (i.e. UCH really) and "plan" have no place in the same sentence.