I upgraded on a phone last month and was suppose to change my rate plan as well. After 3+ weeks now, my plan has still stayed the same. When I've attempted to change my rate plan on self-serve, an error message pops up saying "You can’t change your plan or add-on’s within 14 days of activating or upgrading your phone. Please try again after the 14 day period is over to make this change." It however has been well over 20 days now that I've upgraded/activated.
Do you have any more details? What does it tell you when you try. Are you trying via a browser? You can change once per Billing cycle so if you already have changed it then you have to wait. Otherwise you can ty clearing browser cache and going into private / incognito mode of your browser and try again let us know.