Assuming that you're grouping on account or something of the sort, sort the data on the start date descending. Then you'll use a couple of formulas something like this:

{@InitVars}
WhilePrintingRecords;
NumberVar currentRate := 0;
NumberVar prevRate := 0;

Put this formula in the Account group header - nothing will display, but we need it there to initialize the variables.

{@CalcRates}
WhilePrintingRecords;
NumberVar currentRate;
NumberVar prevRate;
if currentRate = 0 then
currentRate := {rate field}
else if prevRate = o then
prevRate := {rate field};

Then you'll need a third formula to do your actual calculation. It will look something like this:

{@RateChange}
WhilePrintingRecords;
EvaluateAfter({@CalcRates});
NumberVar currentRate;
NumberVar prevRate;
<do your calculation here>

NOTE: Because of the way these need to be calculated, you may only be able to use the final formula in the Account group footer.

-Dell