Maybe it was because there was no more APH rates in the system. So they would be stuck with one night at APH rate. I know it sounds stupid but thats how they might treat it.
Today I just added another day to our May trip (FLO rate) without cancelling anything. I asked before we started anything. I asked if I would have to cancel then rebook. Then have the deposit refunded then charged again. I was told no, she just looked up the reservation and asked if I wanted the day before or after. Then looked it up, said the day is still available, gave me the price of the night (still FLO rate), then added it. She then asked if I would like an email with the extra night showing. She sent it right away and it still has the same reservation number too.
In the past when I book a rack rate far in advanced then change as the rate drops, then they cancel and refund. Then they rebook and charge again for the new reservation. I have not tried to add or subtract close to the time of vacation.
Yes, maybe their booking process is weird by the way they treat the different rates.