Why is it so hard to change a plan when you have a flex box?

I was able to see promotions on the website before I got the Xfinity Flex but after I received it I couldn't see plans anymore. So the only way I can see promotions is to return the flex box. So I have to return the equipment, change my plan and then order another flex box. Seems like way too much extra just to see plans. It's a waste of time for me and a waste of money because comcast is getting back a used flex box and have to send another one out.