You can definitely get it in two weeks, no need to cancel. The expedited procedure definitely exists and it works pretty well if you do it right. The important thing is to go to the prefecture or sub-prefecture with ALL your documents at once - expired passport, recent proof of address, ID photos that meet the standards, birth certificate extract, and if possible a letter justifying the urgency (plane ticket, hotel reservation, etc.). Without that you'll just be making the trip for nothing.
The cost is the same as for a regular passport, except you have extra fees for the acceleration - usually around 30-45 euros more, but it varies by region. The urgent timeline is often 5-7 business days, sometimes less if you ask nicely and your file is complete. The wait on site can be long, so go early in the morning or on a weekday rather than Friday.
One trap people forget: you really have to check your prefecture's opening hours (they often close in the afternoon) and call ahead if possible to see if they can see you quickly. Some even require an appointment now. And be careful to make sure your photos meet the new standards - old passport photos from 5 years ago often don't pass anymore. If you have any doubts, buy new photos at the pharmacy, it's quick and cheap.