Riccardo,
The "old" PM PrepCast "expires" on July 31st in the sense that you should no longer use it to prepare for your PMP exam. This is because that version is based on PMBOK 4 and after July 31st the exam will be based on PMBOK 5. So if you want to properly prepare for the exam after July 31st, then you should buy and use the new prepcast.
However, the 35 Contact Hours that you have earned by watching The "old" PM PrepCast are still valid. Contact hours never expire. So purely theoretically you could watch the "old" PrepCast and still take the "new" exam. But I cannot say how successful you will be, because the PMBOK Guide has changed quite a bit.
Please read this article here where I give suggestions on how to "upgrade" yourself to the new exam:
www.project-management-prepcast.com/inde...ok-guide-5th-edition