If you want to try and use saves from a older version of the game (like 2.4 with 2.5) load and save them in a unpatched 2.5, meaning no transition patch installed as przemo said
right above. Them move the save or patch the game.
There's a unpatched 2.5.5 to use in
przemo's MEGA folder. Don't think you can use old saves with 3.0 though, it just doesn't recognise them.
Edit -
For just one of the patches on it's own look
here, or in przemo's MEGA folder again. If you pick the
modded one then you'll also need
this fix. Patches are preinstalled to the games in the opening post (but not the fix).