I wouldn't say that anytime you modify but often there is some minimum stay period that you are "violating" with the modification so that you have to reprice the stay. Often times, this is on a Sunday night (at a "city" property) - separately its priced at a "weekday" rate while if in conjunction with Friday / Sat nights, it's at the "weekend" rate. In the OP's case, I suspect that since the period in question includes NYE, the original reservation waived the "surcharge" since it was a 4 night stay, while the modification did not waive that "surcharge" since it is only 2 nights.