Changing the type of titre de séjour is tricky, though not impossible. I would speak to the school first of all, to see if they can handle the paperwork on their side so that you will have your "official sponsorship" in hand before you put in for renewal of your carte de séjour.

You might also pay a quick visit to the local préfecture (where you go to renew your card) and ask the question now. There will be no record of your visit, and with any kind of luck, they'll give you a list telling you what documents they will want to renew your card and change your status. If you don't like the answer they give you now, check back with them in September after you've contacted the EHESS and have some paperwork from them to produce.

It's a strange system, but sometimes it actually works in your favor.
Cheers,
Bev