According to this Consumer Reports video episode posted on March 24th, older Model S cars (pre-Autopilot) will NOT be compatible with version 7 software release. This is the upcoming release that Elon said will contain a full UI redesign along with other features. If this is true, and it would be a big deal, Tesla will be sunsetting software releases to the pre-Autopilot cars beginning with version 7. He makes the statement in a very sure way, which leads me to believe he obtained his information directly from Tesla.
Here is the video starting at 9:54 where the statement is made:
Talking Cars with Consumer Reports #64: Tesla and the self-driving car - YouTube
Here is the video starting at 9:54 where the statement is made:
Talking Cars with Consumer Reports #64: Tesla and the self-driving car - YouTube