MediaSentry was bought out by Artistdirect in 2009 and is now a subsidiary of Artistdirect called Peer Media Technologies. Does the email mention those names at all? I don't believe the name MediaSentry is used anymore due to bad publicity regarding their tactics, and possible legal problems in their investigations.
Are you sure this message is from your ISP, and not some sort of phishing scam?
Best to contact your ISP directly regarding this. And changing IP address will do absolutely nothing to hide you from your ISP.