What did you book originally and what did you book in the new one? Do you have the original e-ticket email for the original booking so you know how much you paid in avios and taxes/fees/charges?

The FTV should hold the avios and cash you originally paid, and as you say this is applied to the value of a new reward booking you make. If it is the same route then the avios may differ if going off-peak to peak, but the taxes/fees/charges should be more or less the same.