Demo's out-of-date

[Burning Fish]

I am working on doing a video-review for Episode Two for a Dutch Half-Life 2 website. I recorded a lot of demo-files over the past week when replaying Episode Two and I was planning to start capturing them with Fraps this weekend. But - Valve seems to have released an update which changed the protocol version of the Source engine? Whenever I try to play one of the demo files, it says:

ERROR: demo network protocol 11 outdated, engine version is 12 
Failed to read demo header.

Is there a way to maybe edit the demo headers or install Episode Two without updating? I tried that last method and it was unsuccesful, but maybe I was doing it the wrong way.
Can anyone please help me with this?