Easiest method is to reinstall the old patch version of the game with a full installer. This also ensures the replay will run smoothly (and that's why I keep most installers from old patches). And it's not even that bad, since the full installation takes seconds.
Otherwise, you have to edit the old replay (with any hex editor,
https://hexed.it/ for instance) in order to force the game to recognize it as a legitimate replay of the current version.
Open a replay played under the current version of the game and the old one you want to watch, and look for the differences in the beginning of the code which are relevant to the version (can't remember exactly where they are sadly, and the old gamereplays.org forum post has vanished with the entire website I think). Replacing the old replay's lines with the new ones will work, that is to say you will be able to launch the replay ingame.
But that doesn't take into account the many bugs which can occur as you will be watching, since the game will try and use all the new units abilities visual effects etc... whereas your old replay may have a lot of old things inside.