~via BB (wap.rabroad.com)~I don't know about upgradeing on line, but I've up graded many times on the phone. Just did It Thursday for my niece who is on my account. And I've never had to pay right away. Not one time have I had to pay. They have everytime just added it to my next bill.
Now with that said I need to say, I've been with them for a good while, 4 lines, never ever been late with a bill and I've never ever gone over in my plan. So that may be why they extend me credit.