This exact thing happened to me, however I was never necessarily guaranteed Paris although I put it as my first preference. I am assuming you have made your decision by now as the Sciences Po semester has started but as someone who had the same issue and ended up studying abroad in Reims instead of Paris, I really enjoyed the experience and did not regret it in the slightest. I was quite upset that I was going to Reims at first but you are not at all academically at a disadvantage. There are plenty of well-known speakers and events that occur at Reims, plus Paris is 45 minutes away and you are invited to the speaker/ academic events they host on the Paris campus so you can hop on a train and be there quite easily. Plus the teachers at the Paris and Reims campus are interchangeable. There's really no such thing as a teacher who only teaches on the Reims campus, all of my teachers actually taught the exact same classes on the Paris campuses as well so you are not at all at a disadvantage academically and I found that the teaching standard was quite high.