There's no need to wonder about schedule changes - you just have to read their policy. Airline algorithms automatically put people on the next best flight if a flight is cancelled (that would be the case no matter if the fare was purchased from airline or from Priceline). If the schedule change is too adverse, Priceline advises the purchaser to call them. That seems to me to imply "they will work with you". What else can they "guarantee"? They can't control an airline from making its own decisions re: schedule changes or cancellations. Buying many months in advance comes with some risk no matter what...what's wrong with getting a low price while you're at it?
From Priceline's FAQs:
"Questions about schedule changes?
Airlines, for various reasons, will adjust your itinerary, most commonly flight times, prior to your departure. If your itinerary undergoes a schedule change, we will notify you of the changes via email. In addition, we recommend that travelers confirm their flights and check-in location directly with the airline prior to each departure.
If a schedule change results in an unacceptable itinerary, please call us as soon as possible."