All my calls to the e-service center from the US have had incredibly long hold times. When I called the e-service center from Japan (using the local number) I had to call within a fairly limited window of business hours, and the line was often busy, but when I got through the wait was short.

I was in a similar situation to the OP, where I just wanted to change an existing reservation (and go from Y to J -- more $$ for CX). I was willing to pay the change fee, but the CX reps were helpless. In the end, all they could do was cancel my original ticket, which I got refunded less the cancel fee. I had to book the new J ticket online.