How to play old demos? (CL_ParseClassInfo_EndClasses: CreateDecoders failed)

Sometime between February and late April, something that updated caused my (not so) old demos to stop working. Any demo I made from before April gives me the good old “Host_EndGame: CL_ParseClassInfo_EndClasses: CreateDecoders failed.” error. Here’s what I get in the console:

So, I guess my real question here is, is it possible for me to get these demos to play without having to download an older version of Garry’s Mod?