Why can't I see options or make any changes to my plan online?

I have never been able to manage any plan changes online.  Every time I attempt to do so the only message I receive is"Our plans have changed. Our current plans are more packed with features than the one you're on. If you want to change your plan, select similar services and compare them with what you have now. Nothing is final until you complete your order, so start exploring and customize your plan to fit your lifestyle."

If at click customize the only option provided is to add mobile services.  I would like the ability to shop and adjust my plan without calling into customer service and having to sort out what is best on the phone.  It would be nice to be able to sit in front of the computer and sort out what is best.  How can I make adjustments to my plan?