Upon more research, it seems AT&T backdate your plan change therefore you will be paying a full price for the new plan and leaving you with only 1.1 GB the rest of the cycle. If that is the case, I've been misinformed again by the bright people known as AT&T reps. So does AT&T still backdate the bill cycle or do they pro rate both plans for those that have tried it.
Upon even more research, I saw that Verizon will prorate both plans but calculates your data allowance daily. Meaning if you used 1GB in 15 days and you are on the 30 days 1GB plan and change your plan mid cycle, you will get hit with a 500MB overage charge, since you are only allowed 500MB in the 15 days period. So common advice was to backdate the higher plan to avoid overage charge. In conclusion, there is no loophole if this is how they calculate the prorated bill.
I heard AIO does not do that and allowed you to use the new plan's entire data allowances when you change plan.
In conclusion, ignore this thread. I was misinformed by the AT&T reps. The best way to avoid overage charge is to BACKDATE the new plan, otherwise you will likely incure an overage charge since that is calculated as a daily allowance.