Rental and data benefits will be prorated, if plan change is done in the middle of bill cycle.
Example, if total data benefit offered by your plan is 10 GB and if bill plan is changed 15 days post the start of bill cycle, then pro-rated data benefit (FUP) applicable for the 15 day period will be 5 GB only.
Until now only rental charge was prorated but user got full data. Not sure if it's implemented.