Playing an "old" .dem file

[tronx5]

recently garrys mod has been updating constantly, making my old .dem files useless but overnight it got patched again after i recorded an amazing momment, and i cant play it

after getting loads of normal errors i get this

Host_EndGame: CL_ParseClassInfo_EndClasses: CreateDecoders failed.

i have no idia what it is / how to fix it, i know this is a common question and i know there is a way to fix it.

[tiedie]

If it is anything like TF2 the only way to play them is to either wait for Valve to release their new demo player (talked about almost 2 years ago?) or backup the game files before updates (gcf's I think)