There doesn't appear to be a way to cancel or change the SIMO plan via the website.
If I call 1913 there is an automated main menu message on that number with 4 options, each of which goes to another automated message and back to the main menu. The option for upgrades checks my number and says I can get an upgrade, and tells me to go to the website, where as I've said there is no way to cancel the SIMO plan.
There is no way that I can see from the 1913 number to speak to a human. How can I speak to somebody?
Thanks