Since this is solved through OP calling the PM line, I want to make a point. OP got lucky that they found someone that was willing to help on the other end, most likely because they are PM. BE are not eligible for C+ or F. I am guessing that since they were able to get seat assignments, that they reissued tickets in MC and will now be eligible for those upgrades. OP, are you now showing your names on the upgrade lists?
My wife refuses to pay an extra $50 a ticket on $160 RT tickets to see her family, so she books BE a lot more than I would. I have only been able to switch things up twice, once when something came up that we had to cancel our RT and book LAX>ORD then home where they were kind enough to reissue in MC to make the change the day before and another was in a SL when I was sitting in a middle seat in the back and they worked their magic to get me an aisle C+.
Point is, dont ever count on it.
And yes as others mentioned, you are on standby but there are seats blocked out for you and you are always at the top of the list with the others who have space blocked out as well.