Change a payment agreement

I set a payment to be due tomorrow, which I needed to try and change- I called once, and he said he would switch me over to take care of it but while I was waiting, I lost the call. When I had a better signal later on, I called back The girl I spoke to told me I could not change a payment agreement- I know for sure I have done this many times, since I got the phone in 2006 I have never had service disconnected. What I want to know is, Has something changed where I cannot add a few days, and my only option is to just let it be shut off? Does anyone know?
I am not complaining, just was wondering. I have always got awesome customer service in the past, never a problem with that. Thanks!