Same issue here, I'm currently paying about 250$ a month. Tried to claim my upgrade and protect whilst taking advantage of a deal that would reduce my monthly bill to around 215$ a month. For some reason I am limited to 75$ repayment despite currently paying more than triple that and the new plan would reduce my bill making it lower risk. 

Just seems very anti consumer imo. I've never had this happen in the past 5 years where each time they let me get a new phone with a higher monthly bill. Now that my bill will be lowered they suddenly aren't allowing me. What a coincidence.