Are you getting a specific error message when trying to change your plan on My Account? Did you have the chance to try this from a different browser?
Can you also let us know if you're using a monthly or prepaid service?
FidoPierre, Just tried on another browser, that's the message i got "We're working on fixing it as quickly as we can. In the meantime, we've saved your selections so you can call customer care or use our Live Chat to help finish your order."
@MrC thank you for giving this a try and sharing this information.
In this case I'll send you a PM directly on here so we can take a closer look at your account together.